3o8 SCOTT. [Vol. XI. 



C. The Milk Dentition (PI. XXI, Fig. 5) of the lower jaw is 

 not represented in the collection, and that of the upper jaw only 

 partially so. The second mi,lk molar is very elongate and 

 compressed and consists of three elements, the protocone in 

 the middle, with a basal cusp in front of and behind it, all of 

 which have acute apices and trenchant edges ; the internal 

 cingulum is strongly developed and forms a prominent ridge. 

 This tooth, aside from the cingulum, is strictly in the " tri- 

 conodont stage," though, of course, no phylogenetic signifi- 

 cance can be attributed to this fact. The third milk molar (<£) 

 is very peculiar and not quite like the corresponding tooth of 

 any artiodactyl with which I have compared it, though most 

 resembling that of Gelociis, Leptomeryx, and the tragulines, and 

 thus is quite distinct from the type of d3_ in the existing Pecora. 

 As in the former and in very many ancient genera, such as 

 Dichobime, Dichodon, CcBuotheriiim, etc., the crown bears three 

 external cusps, the protocone and tritocone and in front of the 

 former an anterior basal cusp, for which I have proposed no 

 name, because of the comparative rarity of its occurrence. 

 The tritocone is shaped precisely like the external crescent of 

 p4 , while the protocone is of compressed conical form. The 

 postero-internal cusp, or tetartocone, is likewise crescentic, and 

 thus the hinder half of the tooth is like the entire crown of 

 p4. So far, this description would apply equally well to 

 Leptomeryx, the difference between the two genera consisting 

 in the great prominence of the inner cingulum on the anterior 

 half of the crown in Protoceras, which is thickened and elevated 

 at two points to form two incipient cusps ; of these the pos- 

 terior, opposite the protocone, is of course the deuterocone, 

 while the anterior one has not been named. 



In the Pecora ([3^ has the form of a permanent molar. I 

 have elsewhere shown (No. 10, p. 370) that Rutimeyer has 

 probably attributed too great taxonomic value to this fact, since 

 in the same family both types of f^ may occur, as, for example, 

 in Oreodon and Mejychyus. Yet, nevertheless, the feature is 

 important and it is not without significance that while Proto- 

 ceras resembles the Pecora in many details of skull structure, 

 the dentition, and especially the milk teeth, arc those of the 

 more ancient genera. 



