Studies of the Mammalian Dentition.–I. The Teeth of Centetes ecaudatus and its Allies.

Abstract

Summary..A comparison in Centetes ecwudatus and related forms of each tooth with those adjacent to it has led to the following conclusions:– In all Zalambdodonts there is in the cheek region a progressive simplification on either side of an anticlinal tooth. Posteriorly, in the upper jaw, the simplification involves principally a reduction of the posterobuccal part of the tooth, including the posterobuccal root and the corresponding cusps on the crown, and a shortening of the posterior edge of the tooth. Anteriorly, in the upper jaw, the lingual and posterobuccal roots approach and unite, the main cusp takes up a central position, the lingual ledge moves to the posterior end, the buccal cusps are reduced to a posterobuccal swelling near the base, the anterior ridge of the main cusp disappears, and the anterior cusp becomes rudimentary. These sequences, occurring in all species, affect different teeth in different species. The anticlinal tooth can vary from pd4 to m2, and the amount of difference between adjacent teeth differs according to the species. The simplification sequence is similar in the premolars and the milk‐molars, but there are differences between the two series, the premolars being usually larger and broader, with blunter main cusp and smaller accessory cusps. The molars resemble the milk‐molars rather than the premolars, and must be regarded as the posterior members of the milk‐molar series. At the posterior end of the lower jaw there is often no reduction process, and the anterior reduction takes a different form in the lower jaw from the upper. The metaconid merges into the slopes of the protoconid, the paraconid takes up an anterior position and becomes low, the hypoconid and entoconid disappear, and the hypoconulid remains as a small posterior cusp. In both jaws the anterior cheek‐teeth are formed on the same plan. There are two roots, a central main cusp, and small anterior and posterior accessory cusps. The canines and incisors are modifications of this basic form: in the canine the roots are usually united, the main cusp is very large, and the other cusps are small or absent; in the incisor the cusps are flattened and more equal in size. Additional cusps sometimes appear, especially a labial cusp in the lower jaw and an anterohngual cusp in the upper jaw. Variations in the shape of one tooth are frequently correlated with corresponding variations in other teeth. With regard to variation and evolution, teeth are part of a system, and not independent units. The enlargement of the permanent canines in the male Centetes is accompanied by a reduction of the anterior premolars. Similarity of pattern of adjacent teeth is almost certainly indicative of homology rather than of convergent evolution. This makes possible a return to the premolar‐anatomy theory in its simple form, i. e., the homo‐logizing of molar cusps by discovering the mode of transition from the simple anterior tooth to the molar. The similarity of the anterior teeth in both jaws makes possible a morphological comparison of upper and lower molars. Such a comparison indicates that (a) paracone=protoconid=main cusp of the canine and other anterior teeth, (6) the lingual side of the upper tooth=the buccal side of the lower one and vice versa, and (c) the trigonid is not a more ancient part of the lower molar than the talonid.