INSTITUTE OF SCIENCE, PHILADELPHIA. 45 



In the premolars of Dicotyles there is an evident disposition to continue 

 the appearance of the true molars, being less well developed forms of the 

 same pattern, whereas in Platygonus they exhibit but a single pair of well- 

 produced lobes with a well-developed and distinct basal ridge. 



The true molars of Platygonus are narrower transversely in proportion 

 to their extent fore and aft than in Dicotyles. 



The last premolars of Dicotyles especially are less well developed forms 

 of the succeeding molars, with four constituent lobes to the crown. In 

 Platygonus they consist of a pair of well-developed lobes with a distinct 

 and wide tubercular heel behind and a narrower basal ridge in front. 



The second premolars of Dicotyles are more reduced patterns of the 

 molars, while in Platygonus they are smaller repetitions of the last premolars. 



In the first upper premolars of Dicotyles labiatiis, there are usually three 

 constituent lobes to the crown, with a fourth more rudimental one. Those 

 of Platygonus repeat the condition of those behind reduced in size. 



In the first lower premolars of Dicotyles, the hinder lobes of the succeed- 

 ing teeth are reduced into the condition of a strong tubercular heel and the 

 anterior pair of lobes are completely connate. 



In Platygonus, the first lower premolars are like the succeeding ones, 

 with the constituent lobes quite distinct. 



The canine teeth of Platygonus are like those of Dicotyles, but are pro- 

 portionately more slender and less robust, and they are laterally less 

 divergent. 



The number of incisors in Platygonus is the same as in Dicotyles, h\x\. the 

 third or last of the series appears to be later retained in the latter ; for in 

 the adult mandible of the former it is absent and its alveolus is obliterated, 

 and in a younger specimen both alveoli are so shallow as to render it prob- 

 able the teeth had been shed. 



The incisors of Platygonus are very much smaller than in Dicotyles, 

 barely more than half the size. 



The young skull of Platygonus, mentioned in the early part of the 

 present article, is a little longer and narrower than the adult specimens 

 above described. The zygoma below the orbit is almost flat on the outer 

 surface from the absence of the eversion of the upper and lower borders in 

 the adult skulls. The angle of the mandible is less produced downward, is 

 less everted and has the bordering ridge less well developed. 



Comparative measurements of the skulls of Platygonus compressus, adult 



and young, and of Dicotyles labiatus, adult : 



Platygomjs. Dicotyles. 



Length of skull from top of inion to end Aduit. Young. Aduit. 



of nasals in median line, . . . 292 mm. 304 mm. 268 mm. 

 Length from anterior margin of occipital 



foramen to end of premaxillaries, . 268 " 275 " 240 " 



