Axial Furrow Research Articles

The Hypostome of Encrinurus variolaris and its Relation to the Cephalon

AbstractThe hypostome of a specimen of Encrinurus variolaris (Brongniart) from the Wenlock Limestone (Silurian) is described with particular reference to the intricate shape of the lateral wing. This lies very closely underneath the axial furrow of the cephalon in the region of the anterior pit, but it is unlikely that there was a direct articulation of the wing with the pit or the furrow during life.

The Ontogeny of the TrilobiteDalmanitina olini

AbstractThe ontogeny ofDalmanitina oliniTemple (Upper Ordovician) is described on the basis of specimens from Scania and North Wales. The development is similar to that ofD. socialis(Barrande), but good preservation has enabled some detailed changes in the cephalon to be followed. The protaspid and early post-protaspid stages bear an anterior pit in each axial furrow opposite the frontal lobe. The protaspid anterior glabellar furrow becomes joined to this pit by an oblique furrow, and forms with the latter a composite oblique furrow which develops into the adult anterior-glabellar furrow. This mode of development is considered probably to be common to all Phacopids: the significance of the anterior pits in this family is discussed. Preliminary observations are made on the growth-rates of various parts of the glabella, and the frontal lobe is found to grow in length (sag.) at a rate approximately equal to that of the rest of the glabella.

Redescription of the Trilobite Eoharpes Raymond, 1905

From Dr. Archie Lamont :— Following Bancroft's example in dealing with Cryptolithinae, I have for some time considered applying a fringe-character classification to harpids. On this basis, in a manuscript prepared in 1947 I distinguished by name four apparently natural families : (1) with pits of fairly uniform size as in Eoharpes primus (Barrande); (2) with outer rows of larger pits grading into a finer inner pit complex as in Metaharpes amibouéi Lamont; 1 (3) with an outer row of large pits and inner complex of fine pits, but no gradation between the two, as in Scotoharpes domina Lamont; 2 and (4) Harpidae which show a combination of the characters of (2) and (3), as in Harpes macrocephalus Goldfuss. 3 Observations have also been made on the use of facial sutures in classification of Harpacea. In Scotoharpes domina a longitudinal suture has been described which meets a lateral transverse suture on the ala. This suggests a primitive arrangement whereby the longitudinal suture may have corresponded with the boundary between axial and pleural lobes and the lateral suture with the division between the anterior part of the cephalon and the occipital segment. If this is true, it seems likely that in Proparia the lateral suture is preserved, but in Opisthoparia the longitudinal suture only. In " Harpes " flanaganni (Portlock) (Hunterian Museum, Glasgow University, A.40), what seems to be a sensory tubercle occurs in the axial furrow, and from it a narrow suture runs outwards across the posterior half of the ala.

New Species of Calymenidae from Scotland and Ireland

AbstractFour new species of Calymenidae distinctive of horizons of strati-graphical interest are described. Observations of two pairs of anterior pits in the axial furrows of Platycalymene éire sp. nov. and P. duplicata (Murchison), not corresponding with the first lateral furrows of the glabella, indicate a trisegmental origin of the frontal lobe. A possible correspondence of the trilobite hypostome with chelicerae of Arachnids is propounded.

Notes on Two Species of the Genus Ampyx

HEAD-shield acutely triangular. Glabella elongated, subpentagonal, widest across middle, gently convex or subcarinate, without furrows or lobes, having anterior half freely projecting in front of cheeks and produced into a long slender straight horizontal spine grooved along its upper and lower surfaces, fully twice the length of the glabella. Axial furrows nearly straight, diverging anteriorly, with a small deep slit-like pit at their anterior end. Fixed cheeks smooth, broadly triangular, nearly equilateral, reaching half-way up the glabella, with posterior lateral angles slightly curved back. (Free cheeks unknown). Facial sutures slightly sigmoidal. Pleuro-occipital furrow straight, strong, marking off rather broad band-like occipital segment; meso-occipital furrow and segment scarcely defined, smooth, without median tubercle.

IV.—Notes on the Genus Trinucleus. —Part III

The characters of the lateral furrows of the glabella and of the pits which occur in the axial furrows of the head-shield have not had sufficient attention paid to them, and a comparative study of their development in various British species reveals some interesting and important features.It has heen commonly assumed that lateral furrows are absent in some species, and at any rate the omission of any notice of their occurrence in certain cases is difficult to understand. Frequently, moreover, it has been stated that only two pairs are present, even in those species in which the presence of any lateral furrows has been recognized. The nature of the so-called stalk or neck of the glabella has been also generally misunderstood. Mccoy's division of the genus Trinucleus into two genera, Trinucleus, sens, restr., and Tretaspis, was largely based on the presence or absence of this stalk and of lateral furrows; for he says the furrows at the base of the glabella distinguish Tretaspis from Trinucleus, the latter being stated to be without lateral furrows, though the presence of the ‘eye-line’ on the cheek was chosen as the primary distinctive feature of the former. Nicholson and Etheridge, while rejecting Tretaspis as a sub-generic division, mention an additional third pair of furrows at the sides of the anterior swollen portion of the glabella in Trinucleus Bucklandi, though they do not seem to regard them as segmental furrows.

I.—Sedgwick Museum Notes: New Fossils from the Haverford-west District. VIII

Head-shield broadly semicircular, gently convex; posterior margin nearly straight; middle shield projecting slightly in front of cheeks, and rather bent down. Glabella subquadrate, length slightly greater than width, front truncate, nearly parallel-sided, widening a little anteriorly to maximum across first lateral lobes, moderately convex from side to side, rising gradually in height from back to front, where it is somewhat swollen and slightly overhangs anterior margin of headshield; sides marked with three pairs of regular, subparallel, subequal, and nearly equidistant, strong lateral furrows; anterior pair nearly straight, directed a little obliquely backwards, and extending inwards for about one-third the width of the glabella, situated far forward at about one-fourth the length of the glabella, and a little in front of the point of junction of the axial and marginal furrows; frontal lobe short, broad; second pair of lateral furrows nearly straight, arising a little in front of middle of glabella, running in wards parallel to first pair, but a little longer; first lateral lobes with parallel sides; third or basal pair of furrows with inner portion curved back, but not reaching occipital furrow; second lateral lobes of same length along axial furrow as first pair; basal lobes subrhomboidal, rather less than one-third the width of the glabella. Occipital furrow strong, arched forward slightly in middle and at sides. Occipital ring rounded, rather narrow, a little wider in middle. Surface of glabella and occipital ring ornamented with rather coarse tubercles, with smaller ones interspersed. Axial furrows strong, straight, meeting marginal furrows at right angles a little behind first lateral furrows of glabella.

VII.—The Classification of the Phacopidæ

In North and South America and in South Africa there is a considerable variety of types of the Dalmaniles-branch in the Devonian period which have in many cases received distinctive subgeneric names; but species retaining the typical characters of the Silurian forms persist at any rate in North America. Many of these Devonian forms show incomplete second lateral furrows on the glabella, these furrows not reaching the axial furrows and causing a partial coalescence of the two middle lateral lobes. This tendency towards the fusion of the first and second lateral lobes of the glabella is a departure from the perfect segmentation found in typical Silurian members of Dalmanites, and has caused Clarke to group all such forms together into the subgenus or section Synphoria. This type of structure, as Van Ingen has recently shown, is not unknown amongst the Silurian species of Dalmanites in America, but it finds its most pronounced development in Devonian time and occurs in the groups Coronura, Corycephalus, Odontocephalus, and Probolium, all of which are put by Clarke in the section Synphoria. The marginal ornamentation and different processes on the pygidium and head-shield on which these four groups have been founded are scarcely of the same structural importance as the modifications of the glabellar segmentation. As in other families, the spinosity of these forms is the symbol of a last expiring effort before extinction.

I.—Synopsis of the Genera and Species of Carboniferous Limestone Trilobites

Griffithides, Portlock, 1883.—Outline oblong oval; glabella pyriform, gibbous in front, destitute of lateral furrows; basal lobes inflated; cervical lobe broad; eyes small, lunate, smooth; axial furrow clearly defined, outline of free cheek broadly triangular, outer posterior angle sometimes produced into a cheek spine. Thorax with nine segments; pygidium rounded, composed of from ten to thirteen coalesced somites.