166 0. a Marsh— Principal Characters of the Dinoceraia. 



tentorial ridge. The cerebellar fossa is but little larger trans- 

 versely than the medullar canal, and has lateral cavities which 

 may have been occupied by flocculi. The pituitary fossa is nearly 

 round, and of moderate depth. There are no clinoid processes. 

 The brain as a whole resembled that in some Marsupials more 

 than in any other known mammals. Its small size, as the 

 writer has elsewhere shown, is a character apparently pertain- 

 ing to all Eocene mammals ;* the brain-growth during the rest 

 of the Tertiary period having been gradual, and mainly in the 

 cerebrum. 



The teeth in Dinoceras are represented by the following 

 formula : 



Incisors — ; canines — ; premolars — : molars _ ; X2=34. 

 3 ' 1 ' ^ 3 ' 3 ' 



The superior canines are long, decurved, trenchant tusks. 

 They are covered with enamel, and their fangs extend upward 

 into the base of the maxillary horn-cores. There is some evidence 

 that these tusks were small in the females. Behind the canine, 

 there is a moderate diastema. The molar teeth are very small. 

 The crowns of the superior molars are formed of two transverse 

 crests, separated externally, and meeting at their inner extremi- 

 ties. The series is well shown in Plate III, which represents 

 the upper premolars and molars of D. mirahile. The first true 

 molar is smaller in this specimen than the two preceding pre- 

 molars. The last upper molar is much the largest of the series. 



The lower jaw in Dinoceras is as remarkable as the skull. Its 

 most peculiar features are the posterior direction of the con- 

 dyles, hitherto unknown in Ungulates, and a massive decurved 

 process on each ramus, extending downward and outward 

 below the diastema. (Plate V.) The position of the condyles 

 was evidently necessitated by the long upper tusks, as, with the 

 ordinary ungulate articulation, the mouth could not have been 

 fully opened. The low position of the condyle, but little above 

 the line of the teeth, is also a noteworthy character. The long 

 pendant processes were apparently to protect the tusks, which 

 would otherwise be very liable to be broken. Indications of 

 similar processes are seen in Smilodon, and some other Carnivores 

 with long upper canines. With the exception of these processes, 

 the lower jaw of Dinoceras is small and slender. The symphysis 

 is completely ossified. The six incisors were contiguous, and 

 all directed well forward. Just behind these, and not separated 

 from them, was the small canine, which had a similar direction. 

 The crowns of the lower molars have transverse crests, and the 

 last of the series is the largest. (Plate V, figure 3.) 



The vertebrae in Dinoceras, in their main characters, resemble 

 * This Journal, viii, p. 66, July, 1874. 



