58i MB. M. r. WOODWARD ON [May 5, 



1 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 forms 

 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). Group II. (2 genera). 



1. Paracone. 1. Protocone. 



2. Metacone. 2. Paracone. 



3. Protocone. 3. Metacone, 



4. Hypocone. 

 (5. Metaconule.) 



.}■ 



? together. 



1. Protoconid. 1. Protoconid. 



2. Metaconid. 2 or 3. Paraconid. 1 o f„„ii „_ 

 o IT , f Ento. 3 or 2. Metaconid. J ' ^°S^^''^^- 

 3-HeeHjjyp^ 



4. Paraconid. 4. Hypoconid. 



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 



' The condition of this cone in Sorex is uncertain. 



