OCR Text |
Show 1896.] MAMMALIAN DENTITION. 575 cusp. The protocone is the largest cusp, while the para- and metacone form the outer border of the tooth and are separated by a slight notch only. In the foetus, in m. 1 the protocone forms the main mass of the tooth, while the para- and meta-cones form two rouuded external shelves not at present conical; in m. 2 the protocone and a small antero-external paracone are alone visible. The order of formation being:- 1. Protocone. 2. Paracone. 3. Metacone. The lower molars are trituberculo-sectorial, the heel being larger than in Centetes but still very low; here also the protoconid is the first to develop, but it is quickly followed by the paraconid and later by the metaconid; the hypoconid appears as a low backward continuation of the dentine germ, i. e. of the protocone, for it is a direct backward continuation of the base of that cone. The cusps develop in the following order :- 1. Protoconid, 2. Paraconid. 3. Metaconid. 4. Hypoconid. In describing the upper molars of Centetes and Ericulus, I have regarded them, as is usually done, from a tritubercular standpoint; it is perhaps more correct to describe them as presenting a crown consisting of a large V-shaped internal cone sloping gently towards the external border of the tooth in the form of two ridges, which end in an external serrated margin consisting in Centetes of 4 small cusps (Plate X X V I . fig. 34, a & 6, i, 2, 3, 4), two of which (2 & 3) are regarded as the paracone and metacone. Internal the main cone (5) dips sharply down to an internal cingulum, which is slightly expanded posteriorly (7). TALPA EUROPJ3A. It may seem unnecessary to reinvestigate the relationship of the milk and permanent teeth of the Mole, considering that all the details relating to these teeth appear to have been recorded by Spence Bate (1) as early as 1867, and that these have apparently been confirmed by Leche (9), who used the more modern method of serial sections; but unfortunately the former appears to have perpetrated one serious error, and the latter, owing to the fact that the specimens he examined wrhere too young, has failed to rectify it. The point in question is the supposed presence of a needle-like deciduous first premolar in both jaws. Reference to Bate's figures will show that he represents reduced but elongate needle-like predecessors to all the incisor-;, canines, and premolars, that preceding the 4th premolar alone being two-fanged and non-spicular in form. |