OCR Text |
Show 1896.] MAMMALIAN DENTITION. 573 very late elongation of the jaw, not to theretentiou of a primitive character. The Molar Teeth. In the younger of my two specimens m. 1 alone was developed, while in the older stage two molars were present above and below ; in the latter specimen a very strong lingual development of the dental lamina was noticeable in relation to - ^ that connected with m-J- being specially large (Plate X X V . fig. 2Q, d. I.), and a less marked but similarly related structure was observable in connection with m72. The Cusps. The posterior premolars and all the molar teeth belonging to the upper jaw of this genus exhibit a high triangular crown surrounded by a low cingulum, this latter being most marked in the postero-internal region of the tooth (Plate X X V I . fig. 34); the trigon is characterized by the presence of 3 cusps, of which the antero-internal (s) is the dominant and is connected by an oblique ridge with the cusps usually regarded as the paracone and metacone respectively (2, 3)-this tooth apparently presenting an almost pure tritubercular type1. On examining m. 1 in m y oldest specimen, this tooth was found to be composed of a prominent main cone slightly inclined inwards, undoubtedly the protocone of the adult tooth ; while growing out low down from the external surface of this main dental germ were two smaller cones-a slightly more pronounced anterior one occupying the position of the future paracone, and a less developed postero-external cone situated well behind the main cone, i. e. the exact position of the metacone. The order of cusp-development is given below :- 1. Protocone. 2. Paracone. 1 AT 1 u. ^ M" t > JN early simultaneous. In the deciduous 4th premolar likewise the protocone develops first, but here the metacone is in advance of the paracone. The lower molars and posterior premolar are beautiful examples of the trituberculo-sectorial tooth, consisting of a high trigon and a low slightly developed heel; the three cusps of the trigon are pronounced-the protoconid (antero-external) being the largest, the metaconid is next in size and almost hidden by the former as it lies immediately internal to it, the paraconid being the smallest and most anterior cusp. The development of these cones is well seen in m. 1 &m. 2 of my older specimen, and it is at once obvious that the protoconid is the original dentine germ, the other cusps being later outgrowths from 1 The upper cheek-teeth of Hemicentetes should be examined by trituber-culists, for in this genus a complete transition between the triconodont premolars and the trituberculate molars can be seen. |