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Show 584 MR. M. F. WOODWARD ON [May 5, I have adduced reasons elsewhere (29) to support the view that this pre-milk set, i. e. the first in order of time, has been completely lost in the molar region, and that these labial outgrowths of the dental lamina represent the now much reduced milk-dentition-the adult molars belonging to the 3rd or replacing set of teeth, the lingual continuation of dental lamina representing a potential 4th dentition, the post-permanent series. The Molar Cusps. On comparing the details of the molar cusp development in the various Insectivores which I have examined, one is immediately struck with fact that the lower molar cusps in the different form's are more constant in the order of their appearance, the protoconid developing first in every case, than those of the upper molars: these latter fall apparently into two groups-in one the paracone is the first to appear, while in the second it is the protocone which develops first. A closer inspection shows that a similar subdivision of the lower molars can be made ; thus in those forms where the paracone appears first in the upper molars, we find the supposed homologue of this in the lower teeth (the paraconid)' is the last to develop, while those exhibiting the protocone as the first developed cusp above show the paraconid as second or third in order of development below. These facts may be roughly tabulated thus :- Group I. (4 genera). 1. Paracone. 2. Metacone. 3. Protocone. 4. Hypocone. (5. Metaconule.) Group II. (2 genera). 1. Protocone. 2. Paracone. 3. Metacone. ? together. O 1. Protoconid. 2. Metaconid. Ento. Hypo. 4. Paraconid. 3. Heel. 1. Protoconid. 2 or 3. Paraconid. 3 or 2. Metaconid. 4. Hypoconid. ? together. A further examination of these groups reveals the fact that they are separated from one another by a second feature, which is possibly of greater importance than that of cusp ontogeny; I refer to the fact that the members of group I. possess either quadri- or quinque-tubercular upper molars, while in group II. these teeth are trituberculate. It will be seen, then, that in the only living mammals believed to possess unmodified trituberculate teeth (molars and posterior premolars) which have been examined developmentally, the order of cusp ontogeny is in entire accord with the supposed order of 1 The condition of this cone in Sorex is uncertain. |
