| OCR Text |
Show 570 MR. M. T. WOODWARD O N [May 5, Cusps. The molar teeth of the adult belong to the trituberculo-sectorial type, the upper ones have square crowns bearing four cusps, i.e. 3 large equal ones belonging to the trigon and a small posterointernal cusp or hypocone. The lower molars are elongated, consisting of a trigon and a very large heel with two conspicuous cones (hypo- and entoconid); the cones in the lower trigon are not equally developed, the protoconid being much larger than the other two. M y specimen was too young to determine with any certainty the ontogeny of the cones, as only one of the main cones had made its appearance, the dental germ presenting the appearance of a high cone with a large posterior heel (metaconal region) and a slight internal extension. A plan of the dental germ at this stage shows that structure to be roughly triangular, the main and only cone being situated at the anterior extremity and slightly nearer the external border. Prom the position of this cone and from a comparison with the cusp ontogeny as seen in the molar of Talpa, with which it is identical in pattern, I think one may conclude that this single cusp is the paracone, the posterior extension representing the metacone, while the internal shelf indicates the position of the future proto- and hypocone. In the lower jaw the main cusp is antero-external in position, and may be identified as the protoconid; a slight inward extension of the dental germ alone indicates the para- and metaconid, while a faint backward development foreshadows the future heel with its two cones. , The probable order of cusp-development is thus shown:- 1. Paracone. 1. Protoconid. 2. Metacone. 9 f Paraconid. 3. Protocone. [ Metaconid. . TT of Entoconid. 4. Hypocone. 3. j H y p o c o n i d. A general examination of the Shrew's teeth shows that with the exception of r-, and the almost undifferentiated 1st premolar, all the anterior teeth, viz. |-2, i. 3 C. (i. 4), pm. 3, pm. 4 ^ . ^ ^ l. - \\, o) pin. ^ labially situated enamel-germ of a predecessor, some of these being in a highly developed condition, possessing marked dentinal germs but no calcification. These structures, from a comparison with the teeth of other Insectivora, must be regarded as vestiges of that earlier developed set of teeth the milk-dentition : this homology is very striking when they are compared with the reduced milk-teeth seen in Erinaceus, and is rendered still more certain when we remember that there is indication of no other tooth replacement in the Soricidce. The relations of ^ are very confusing, for the large develop- |
