Hyoid Skeleton Research Articles

Anatomía del sistema digestivo del pescado blanco Chirostoma humboldtianum (Teleostei: Atherinopsidae)

Background. Chirostoma humboldtianum (Valenciennes, 1835) is the first ichthyic species scientifically described in Mexico. Diversification processes of the genus Chirostoma Swaison, 1839 are related to the specialization of its mandibular apparatus; yet few studies exist regarding the digestive system in Atherinopsidae. In particular, the anatomical aspects of its mandibular, branchial, and hyoid skeleton, mandibular and pharyngeal dentition, and visceral musculature are superficially described. Goals. Analyze and describe the general organization of the digestive system of Chirostoma humboldtianum as part of its basic biology, in order to reveal the feeding processes useful for its conservation or culture. Methods. Adult specimens, previously fixed with formaldehyde and preserved in ethyl alcohol, were obtained. Their visceral skeleton and musculature and digestive tract were analyzed by dissections. Results. Major characteristics include presence of lower interpharyngeal cartilage, extensive expansion of levator operculi muscle over external opercular surface; presence of tendinous complex of A1 section of the adductor mandibulae muscle, which relates this section with the lacrimal and anguloarticular bones; presence of an additional section of pharyngoclavicularis externus muscle; insertion of retractor dorsalis muscle in vertebrae 2 to 4, absence of a stomach; and presence of a dental plate in each ceratobranchial 4. Conclusions. The organization of the digestive system of C. humboldtianum corresponds to an Atherinomorpha anatomic pattern, because of the absence of a stomach, among other characteristics; it particularly corresponds to the Atherinopsidae anatomical pattern due to its visceral skeleton organization. This organization, in particular, of the mandibular and pharyngeal adult dentition is characteristic of a carnivorous fish that includes other teleosts in its diet.

The remarkable vocal anatomy of the koala (Phascolarctos cinereus): insights into low-frequency sound production in a marsupial species.

Koalas are characterised by a highly unusual vocal anatomy, with a descended larynx and velar vocal folds, allowing them to produce calls at disproportionately low frequencies. Here we use advanced imaging techniques, histological data, classical macroscopic dissection and behavioural observations to provide the first detailed description and interpretation of male and female koala vocal anatomy. We show that both males and females have an elongated pharynx and soft palate, resulting in a permanently descended larynx. In addition, the hyoid apparatus has a human-like configuration in which paired dorsal, resilient ligaments suspend the hyoid apparatus from the skull, while the ventral parts tightly connect to the descended larynx. We also show that koalas can retract the larynx down into the thoracic inlet, facilitated by a dramatic evolutionary transformation of the ventral neck muscles. First, the usual retractors of the larynx and the hyoid have their origins deep in the thorax. Secondly, three hyoid muscles have lost their connection to the hyoid skeleton. Thirdly, the genioglossus and geniohyoid muscles are greatly increased in length. Finally, the digastric, omohyoid and sternohyoid muscles, connected by a common tendinous intersection, form a guiding channel for the dynamic down-and-up movements of the ventral hyoid parts and the larynx. We suggest that these features evolved to accommodate the low resting position of the larynx and assist in its retraction during call production. We also confirm that the edges of the intra-pharyngeal ostium have specialised to form the novel, extra-laryngeal velar vocal folds, which are much larger than the true intra-laryngeal vocal folds in both sexes, but more developed and specialised for low frequency sound production in males than in females. Our findings illustrate that strong selection pressures on acoustic signalling not only lead to the specialisation of existing vocal organs but can also result in the evolution of novel vocal structures in both sexes.

Cranio-facial remodeling in domestic dogs is associated with changes in larynx position.

The hyo-laryngeal complex is a multi-segmented structure integrating the oral and pharyngeal cavities and thus a variety of critical functions related to airway control, feeding, and vocal communication. Currently, we lack a complete understanding of how the hyoid complex, and the functions it mediates, can also be affected by changes in surrounding cranio-facial dimensions. Here, we explore these relationships in a breed of domestic dog, the Portuguese Water Dog, which is characterized by strong cranio-facial variation. We used radiographic images of the upper body and head of 55 adult males and 51 adult females to obtain detailed measures of cranio-facial variation and hyoid anatomy. Principal components analysis revealed multiple orthogonal dimensions of cranio-facial variation, some of which were associated with significant differences in larynx position: the larynx occupied a more descended position in individuals with shorter, broader faces than in those with longer, narrower faces. We then tested the possibility that caudal displacement of the larynx in brachycephalic individuals might reflect a degree of tongue crowding resulting from facial shortening and reduction of oral and pharyngeal spaces. A cadaver sample was used to obtain detailed measurements of constituent bones of the hyoid skeleton and of the tongue body, and their relationships to cranio-facial size and shape and overall body size supported the tongue-crowding hypothesis. Considering the presence of descended larynges in numerous mammalian taxa, our findings establish an important precedent for the possibility that laryngeal descent can be initiated, and even sustained, in part in response to remodeling of the face and cranium for selective pressures unrelated to vocal production. These integrated changes could also have been involved in hominin evolution, where the different laryngeal positions in modern humans compared with nonhuman primates have been traditionally linked to the evolution of speech but which are likely to be multifactorial.

Ligament versus bone cell identity in the zebrafish hyoid skeleton is regulated by mef2ca.

Heightened phenotypic variation among mutant animals is a well-known, but poorly understood phenomenon. One hypothetical mechanism accounting for mutant phenotypic variation is progenitor cells variably choosing between two alternative fates during development. Zebrafish mef2cab1086 mutants develop tremendously variable ectopic bone in their hyoid craniofacial skeleton. Here, we report evidence that a key component of this phenotype is variable fate switching from ligament to bone. We discover that a 'track' of tissue prone to become bone cells is a previously undescribed ligament. Fate-switch variability is heritable, and comparing mutant strains selectively bred to high and low penetrance revealed differential mef2ca mutant transcript expression between high and low penetrance strains. Consistent with this, experimental manipulation of mef2ca mutant transcripts modifies the penetrance of the fate switch. Furthermore, we discovered a transposable element that resides immediately upstream of the mef2ca locus and is differentially DNA methylated in the two strains, correlating with differential mef2ca expression. We propose that variable transposon epigenetic silencing underlies the variable mef2ca mutant bone phenotype, and could be a widespread mechanism of phenotypic variability in animals.

The anatomy of vocal divergence in North American Elk and European red deer.

Loud and frequent vocalizations play an important role in courtship behavior in Cervus species. European red deer (Cervus elaphus) produce low-pitched calls, whereas North American elk (Cervus canadensis) produce high-pitched calls, which is remarkable for one of the biggest land mammals. Both species engage their vocal organs in elaborate maneuvers but the precise mechanism is unknown. Vocal organs were compared by macroscopic and microscopic dissection. The larynx is sexually dimorphic in red deer but not in elk. The laryngeal lumen is more constricted in elk, and narrows further during ontogeny. Several elements of the hyoid skeleton and two of four vocal tract segments are longer in red deer than in elk allowing greater vocal tract expansion and elongation. We conclude that elk submit the larynx and vocal tract to much higher tension than red deer, whereby, enormously stressed vocal folds of reduced effective length create a high resistance glottal source. The narrow, high impedance laryngeal vestibulum matches glottal and vocal tract impedance allowing maximum power transfer. In red deer longer and relaxed vocal folds create a less resistant glottal source and a wider vestibulum matches the low glottal impedance to the vocal tract, thereby also ensuring maximum power transfer.

Jagged-Notch signaling ensures dorsal skeletal identity in the vertebrate face

The development of the vertebrate face relies on the regionalization of neural crest-derived skeletal precursors along the dorsoventral (DV) axis. Here we show that Jagged-Notch signaling ensures dorsal identity within the hyoid and mandibular components of the facial skeleton by repressing ventral fates. In a genetic screen in zebrafish, we identified a loss-of-function mutation in jagged 1b (jag1b) that results in dorsal expansion of ventral gene expression and partial transformation of the dorsal hyoid skeleton to a ventral morphology. Conversely, misexpression of human jagged 1 (JAG1) represses ventral gene expression and dorsalizes the ventral hyoid and mandibular skeletons. We further show that jag1b is expressed specifically in dorsal skeletal precursors, where it acts through the Notch2 receptor to activate hey1 expression. Whereas Jagged-Notch positive feedback propagates jag1b expression throughout the dorsal domain, Endothelin 1 (Edn1) inhibits jag1b and hey1 expression in the ventral domain. Strikingly, reduction of Jag1b or Notch2 function partially rescues the ventral defects of edn1 mutants, indicating that Edn1 promotes facial skeleton development in part by inhibiting Jagged-Notch signaling in ventral skeletal precursors. Together, these results indicate a novel function of Jagged-Notch signaling in ensuring dorsal identity within broad fields of facial skeletal precursors.

Hyoid Skeleton, Its Related Muscles, and Morphological Novelties in the Frog Lepidobatrachus (Anura, Ceratophryidae)

Many traits of the skull of ceratophryines are related to the capture of large prey independently of aquatic or terrestrial feeding. Herein, detailed descriptions of the development of hyoid skeleton and the anatomy of muscles responsible for hyoid and tongue movements in Lepidobatrachus laevis and L. llanensis are provided and compared with those of other neobatrachians. The aquatic Lepidobatrachus has special features in its hyoid skeleton that integrates a set of derived features convergent with the conditions observed in non-neobatrachian anurans and morphological novelties (e.g., dorsal dermal hyoid ossification) that deviate from the generalized pattern found in most frogs. Further, reduction of fibers of muscles of buccal floor, reduction or loss of hyoid muscles (m. geniohyoideus rama lateralis, anterior pair of m. petrohyoideus posteriores), small tongue, and simplified tongue muscles are also morphological deviations from the pattern of terrestrial ceratophryines, and other aquatic ceratophryids (e.g., Telmatobius) that seem to be related to feeding underwater. The historical derived features shared with Chacophrys and Ceratophrys involved in megalophagy are conserved in Lepidobatrachus and morphological changes in the hyoglossal apparatus define a unique functional complex among anurans.

Vocal tract motor patterns and resonance during constant frequency song: the white-throated sparrow

Bird song is a complex behavior that requires the coordination of several motor systems. Sound is produced in the syrinx and then modified by the upper vocal tract. Movements of the hyoid skeleton have been shown in the northern cardinal (Cardinalis cardinalis) to be extensively involved in forming an oropharyngeal-esophageal cavity (OEC), which contributes a major resonance to the vocal tract transfer function. Here we report that a similar relationship exists between the volume of the OEC and the fundamental frequency in the white-throated sparrow (Zonotrichia albicollis) whose song, unlike that of the cardinal, consists of a series of almost constant frequency notes. Cineradiography of singing sparrows shows that the oropharyngeal cavity and cranial end of the esophagus expand abruptly at the start of each note and maintain a relatively constant volume until the end of the note. Computation of the vocal tract transfer function suggests a major resonance of the OEC follows the fundamental frequency, making sound transmission more efficient. The presence of similar prominent song-related vocal tract motor patterns in two Oscine families suggests that the active control of the vocal tract resonance by varying the volume of the OEC may be widespread in songbirds.

Songbirds tune their vocal tract to the fundamental frequency of their song.

In human speech, the sound generated by the larynx is modified by articulatory movements of the upper vocal tract, which acts as a variable resonant filter concentrating energy near particular frequencies, or formants, essential in speech recognition. Despite its potential importance in vocal communication, little is known about the presence of tunable vocal tract filters in other vertebrates. The tonal quality of much birdsong, in which upper harmonics have relatively little energy, depends on filtering of the vocal source, but the nature of this filter is controversial. Current hypotheses treat the songbird vocal tract as a rigid tube with a resonance that is modulated by the end-correction of a variable beak opening. Through x-ray cinematography of singing birds, we show that birdsong is accompanied by cyclical movements of the hyoid skeleton and changes in the diameter of the cranial end of the esophagus that maintain an inverse relationship between the volume of the oropharyngeal cavity and esophagus and the song's fundamental frequency. A computational acoustic model indicates that this song-related motor pattern tunes the major resonance of the oropharyngeal-esophageal cavity to actively track the song's fundamental frequency.

Temporal requirement ofHoxa2in cranial neural crest skeletal morphogenesis

Little is known about the spatiotemporal requirement of Hox gene patterning activity in vertebrates. In Hoxa2 mouse mutants, the hyoid skeleton is replaced by a duplicated set of mandibular and middle ear structures. Here, we show that Hoxa2 is selectively required in cranial neural crest cells (NCCs). Moreover, we used a Cre-ERT2 recombinase system to induce a temporally controlled Hoxa2 deletion in the mouse. Hoxa2 inactivation after cranial NCC migration into branchial arches resulted in homeotic transformation of hyoid into mandibular arch skeletal derivatives, reproducing the conventional Hoxa2 knockout phenotype, and induced rapid changes in Alx4, Bapx1, Six2 and Msx1 expression patterns. Thus, hyoid NCCs retain a remarkable degree of plasticity even after their migration in the arch, and require Hoxa2 as an integral component of their morphogenetic program. Moreover, subpopulations of postmigratory NCCs required Hoxa2 at discrete time points to pattern distinct derivatives. This study provides the first temporal inactivation of a vertebrate Hox gene and illustrates Hox requirement during late morphogenetic processes.

Dentition and dentigerous bones in juveniles and adults of Polypterus senegalus (Cladistia, actinopterygii)

Tooth types, their arrangement and the mode of tooth replacement were studied in juvenile and adult specimens of Polypterus senegalus by means of scanning electron microscopy of cleared and stained specimens as well as sections. All the dermal bones of the oropharynx are almost completely covered with teeth except for the angulare. The same is true for the branchial apparatus where only the hyoid skeleton is toothless. The teeth are uniformly monocuspid and conical, but can be classified according to shape and size into three types. These types and the mode of tooth replacement are characteristic for each dermal bone. In some of the jaw bones each tooth possesses a lingually situated replacement tooth. This is true for the teeth of the premaxillary, the maxillary, and the dentary which are arranged in a single line, and those of the dermopalatine, the coronoids, and the vomer which are in several lines and graded in size. Replacement teeth of all the other dentigerous elements develop on top of existing pulpal openings, forming an anastomosing common pulpal complex only after resorption of the previous tooth. The tooth plates of the dermal bones of the branchial apparatus are connected by syndesmosis only to the perichondrally ossified and to the cartilaginous or connective tissue material of the elements of the gill-arches. The dentition and its association with the bones of the head in Polypterus senegalus bear resemblances to advanced actinopterygians on the one hand (e.g. differentiation of tooth-types, arrangement), but also some similarities to living Amphibia (anchoring material and mode of replacement) on the other. The accentuation of a single marginal line of large teeth in both, the outer and the inner dental arcade of the jaws is a peculiarity of Polypterus that in a way parallels the derived state of similar monolinear tooth arrangements in Actinopterygii and Tetrapoda.

Stability and Plasticity of Neural Crest Patterning and Branchial Arch Hox Code after Extensive Cephalic Crest Rotation

The extent to which the spatial organisation of craniofacial development is due to intrinsic properties of the neural crest is at present unclear. There is some experimental evidence supporting the concept of a prepattern established within crest while contiguous with the neural plate. In experiments in which the neural tube and premigratory crest are relocated within the branchial region, crest cells retain patterns of gene expression appropriate for their position of origin after migration into the branchial arches, resulting in skeletal abnormalities. But in apparent conflict with these findings, when crest is rerouted by late deletion of adjacent crest, infilling crest alters its pattern of gene expression to match its new location, and a normal facial skeleton results. In order to reconcile these findings and thus identify processes of relevance to the course of normal development, we have performed a series of neural tube and crest rotations producing a more extensive reorganisation of cephalic crest than has been previously described. Lineage analysis using DiI labelling of crest derived from the rotated hindbrain reveals that crest does not migrate into the branchial arch it would have colonised in normal development, rather it simply populates the nearest available branchial arches. We also find that crest adjacent to the grafted region contributes to a greater number of branchial arches than it would in normal development, resulting in branchial arches containing mixed cell populations not occurring in normal development. We find that after exchange of first and third arch crest by rotation of r1–7, crest alters its expression ofhoxa-2andhoxa-3to match its new location within the embryo resulting in the reestablishment of the normal branchial arch Hox code. A facial skeleton in which all the normal components are present, with some additional ectopic first arch structures, is formed in this situation. In contrast, when second and third arch crest are exchanged by rotation of r3 to 7, ectopic Hox gene expression is stable, resulting in the persistence of an abnormal branchial arch Hox code and extensive defects in the hyoid skeleton. We suggest that the intrinsic properties of crest have an effect on the spatial organisation of structures derived from the branchial arches, but that exposure to increasingly novel environments within the branchial region or “community effects” within mixed populations of cells can result in alterations to crest Hox code and morphogenetic fate. In both classes of operation we find that there is a tight link between the resulting branchial arch Hox code and a particular skeletal morphology.

Stability and plasticity of neural crest patterning and branchial arch hox code after extensive cephalic crest rotation

The extent to which the spatial organisation of craniofacial development is due to intrinsic properties of the neural crest is at present unclear. There is some experimental evidence supporting the concept of a prepattern established within crest while contiguous with the neural plate. In experiments in which the neural tube and premigratory crest are relocated within the branchial region, crest cells retain patterns of gene expression appropriate for their position of origin after migration into the branchial arches, resulting in skeletal abnormalities. But in apparent conflict with these findings, when crest is rerouted by late deletion of adjacent crest, infilling crest alters its pattern of gene expression to match its new location, and a normal facial skeleton results. In order to reconcile these findings and thus identify processes of relevance to the course of normal development, we have performed a series of neural tube and crest rotations producing a more extensive reorganisation of cephalic crest than has been previously described. Lineage analysis using DiI labelling of crest derived from the rotated hindbrain reveals that crest does not migrate into the branchial arch it would have colonised in normal development, rather it simply populates the nearest available branchial arches. We also find that crest adjacent to the grafted region contributes to a greater number of branchial arches than it would in normal development, resulting in branchial arches containing mixed cell populations not occurring in normal development. We find that after exchange of first and third arch crest by rotation of r1–7, crest alters its expression of hoxa-2 and hoxa-3 to match its new location within the embryo resulting in the reestablishment of the normal branchial arch Hox code. A facial skeleton in which all the normal components are present, with some additional ectopic first arch structures, is formed in this situation. In contrast, when second and third arch crest are exchanged by rotation of r3 to 7, ectopic Hox gene expression is stable, resulting in the persistence of an abnormal branchial arch Hox code and extensive defects in the hyoid skeleton. We suggest that the intrinsic properties of crest have an effect on the spatial organisation of structures derived from the branchial arches, but that exposure to increasingly novel environments within the branchial region or “community effects” within mixed populations of cells can result in alterations to crest Hox code and morphogenetic fate. In both classes of operation we find that there is a tight link between the resulting branchial arch Hox code and a particular skeletal morphology.

Kinematics of prey processing in Chamaeleo jacksonii: conservation of function with morphological specialization

This study develops a model of prey processing behaviours in lizards of the family Chamaeleonidae based on a kinematic analysis of videotaped feeding sequences in a representative species, Jackson's chameleon (Chamaeleo jacksonii). High‐speed video (200 fields per second) and a computerized image analysis system were used, respectively, to film and quantify sequences of three individuals feeding on crickets. Two behaviours, chewing and prey transport, were identified a priori and compared in detail. Analyses of variance revealed significant differences between the two behaviours in seven of 11 kinematic variables. Patterns of correlation of variables within the entire data set were similar to correlations within each behaviour. A principal component analysis on the kinematic variables provided complete separation of the two behaviours in multivariate space. Chewing is distinct from prey transport behaviour in several ways: (1) chewing lacks extensive posterior movement in the hyoid skeleton while the jaws repeatedly open and close against the prey; (2) there is greater mouth opening in chewing; and (3) the entire gape cycle and its components occur more quickly in chewing. Prey transport involves extensive anterior posterior movements of the hyoid skeleton, which reflect the function of the tongue in transporting the prey from the oral cavity into the oesophagus. In both behaviours mouth opening is primarily associated with lower jaw depression rather than head elevation. Maximum hyoid retraction always occurs after peak gape is achieved. The body and head remain stationary with reference to the background, hence no inertial transport occurs. The kinematic profile of prey transport is compared to published feeding cycles of other iguanian lizards and a generalized model of prey processing in amniotes. Prey transport in chameleons follows the same kinematic patterns as reported for other iguanian lizards. We conclude that chameleons possess a generalized food processing system characteristic of other iguanian lizards, largely unmodified despite the presence of an extraordinary prey capture behaviour and its associated morphology.

Functional morphology of dewlap extension in the lizard Anolis equestris (Iguanidae)

The dewlap is an extendible flap of skin ordinarily folded under the throat. Lizards, particularly those in the genus Anolis, extend their dewlaps during interactions with conspecifics, other lizards, and potential predators. Dewlap extension is effected by movements of elements of the hyoid apparatus. This paper describes the anatomy of the hyoid and associated musculature in Anolis equestris, a large arboreal lizard with a prominent dewlap. A mechanism for dewlap extension is proposed based on results of morphological and experimental techniques. Specializations of the hyoid skeleton for dewlap extension include elongated second ceratobranchials and highly movable joints between the ceratohyals and the hypohyals and between the first ceratobranchials and the body of the hyoid. A well developed M. ceratohyoideus extends between the ceratohyals and the first ceratobranchials of the hyoid apparatus. During dewlap extension, the hyoid apparatus acts as a first order lever. Contraction of M. ceratohyoideus pulls the ceratohyals posteriorly causing the hypohyals and the body of the hyoid to rotate dorsally around the first ceratobranchial/body joints. This movement results in the second ceratobranchials swinging forward and down, unfolding the dewlap. The relative immobility of the first ceratobranchials provides stability to the hyoid apparatus during dewlap extension. A comparison is made of dewlap extension and other hyoid displays.

The mechanism of frill erection in the bearded dragon Amphibolurus barbatus with comments on the jacky lizard A. muricatus (Agamidae).

Amphibolurus barbatus has a threat display which includes the erection of the gular regions as a frill and may also include wide opening of the mouth to display a yellow mouth lining. Frill erection involves protraction, depression, and lateral expansion of the hyoid apparatus. Electrical stimulation of the hyoid muscles and dissection of the hyoid apparatus were used to examine specializations for producing frill erection. Specializations of the hyoid skeleton include the absence of a ceratobranchial II, presence of a synovial joint between the ceratohyal and body of the hyoid, and combined shortening of the entoglossal process and lengthening of the posterior arches. The only apparent specialization of the hyoid musculature is the anterior displacement of the origin of m. hyomandibularis. All of the hyoid muscles are involved in some way in frill erection and the actions of each muscle is described. The characteristic frill erection in the threat display of Amphibolurus barbatus is possible because of the 1:2 ratio of the anterior and posterior parts of the apparatus and the absence of the ceratobrnchial II.

TONGUE MORPHOLOGY AND AFFINITIES OF THE HAWAIIAN HONEYCREEPER MELAMPROSOPS PHAEOSOMA

SUMMARY Melamprosops phaeosoma has a thick straight tongue with an anterior spoon‐like trough, but without any indication of a tube. Its hyoid skeleton has an elongated, flattened basihyale, stout parallel paraglossalia and a concavity along the dorsal surface of the ceratobranchiale. All the tongue muscles could be dissected but the superficial M. mylohyoideus and M. serpihyoideus were too damaged to figure and describe. The M. ceratoglossus and the M. hypoglossus obliquus are large; the latter muscle inserts mainly on the basihyale and ceratobranchiale, but a small number of fibres pass beneath the basihylale. The M. hypoglossus anterior is absent. The morphology of the tongue apparatus of Melamprosops is very similar to that of Ciridops and of Loxops and supports placement of Melamprosops in the Drepanididae. Further it suggests that Melamprosops evolved from an ancestor with a tubular tongue, supporting the suggestion that it evolved from Loxops.

The hyoid, laryngeal and pharyngeal regions of bathyergid and other selected rodents.

The morphology of the hyoid, laryngeal and pharyngeal regions of the following rodent genera was studied: Cryptomys, Bathyergus, Georychus, Heliophobius, Heterocephalus, Ctenomys, Petromus, Thryonomys, Geomys, Cannomys, and Tachyoryctes. A number of morphological conditions unique to bathyergids, and associated with the use of the head and lower incisors in burrowing are described. The conditions include the formation of functional complexes of MM. sterno-geniohyoideus and omo-mylohyoideus, the presence of a unique deep oblique part of M. transversus mandibulae and a strong separate slip of M. platysma myoides pars mentalis. The hyoid skeleton is modified to allow the muscle complexes to act independently of the basihyal bone, and to allow the unusually protrusible tongue to be withdrawn. The nerves of the jugular foramen do not form a true pharyngeal plexus, and their configuration is influenced by the absence of a well developed internal carotid artery in hystricognaths. The morphology of the regions studied indicates a natural grouping of bathyergids, but one in which Heterocephalus is somewhat separate from the remaining bathyergid genera. The grouping of bathyergids and New and Old World hystricognath rodents into a suborder Hystricognatha is supported.

THE HYOID COMPLEX OF THE PALM SQUIRREL, FUNAMBULUS.

AbstractThe hyoid apparatus of the squirrel, Funambulus consists of basihyal, the immovably fused posterior cornu represented by thyrohyal and the long slender and flexible anterior cornu which is divisible into hypohyal, caratohyal, stylohyal and tympanostyloid synchrondosis. Since the stylomastoid foramen lies posterolateral to the distal tip of the anterior cornu, the hyoid is of the protrematic type. The hyoid apparatus of Funambulus palmarum differs from that of Funambulus pennanti in minor but well marked characters such as a less arched basihyal with smooth shoulders, a small entoglossal process, a trochanter on the lateral side of thyrohyal, a longer hypohyal and a shorter ceratohyal.All the muscles which originate or insert on the hyoid skeleton and their innervation are described. The muscle digastricus of the squirrels is quite characteristic with a well marked intermediate tendon between the anterior and posterior bellies. The muscle jugulohyoideus originates from the paraoccipital process and is well developed, but the stylohyoideus is slender, foreshadowing the condition in gerbils in which only one of these two muscles, possibly the jugulohyoideus is represented.