166 



University of California Publications. [Geology 



In Preptoceras the occipital suture is midway between the 

 lambdoidal crest and the foramen magnum, while in Eucerather- 

 ium the suture is much nearer the crest and gives a relatively 

 greater area to the occipital than in Preptoceras. The lamb- 

 doidal crest overhangs the occiput, forming deep fossae on either 

 side of the median tubercle. These are absent in Euceratherium. 

 In the latter, above the mastoid, where the squamosal, occipital 

 and parietal elements meet, prominent tubercles are formed, 

 from which a buttress-like ridge passes dorsally to the base of 

 the horns. This brings the parietals almost into the plane of the 

 occiput and gives the back of the cranium a scpiare appearance. 

 This angulation is absent in Preptoceras. There is no ridge 

 from the lambdoidal crest to the base of the horns and the parie- 

 tals pass back of the horn-cores to the cranial roof in a uni- 

 formly curved surface, making a deep concavity between the 

 crest and the horn-cores. 



The elements of the basioccipital region are in general 

 broader than in Euceratherium, though the foramina occupy the 

 same positions. The occipital tubercles have not such well 

 defined anterior and posterior areas, the median constriction 

 being less marked in Preptoceras. The paroccipital processes are 

 relatively more robust and higher. A raised posterior elongation 

 of the low-lying tympanic bullae rests against its base more dis- 

 tinctly than in Euceratherium. 



The maxillae are greatly swollen at the sides as in 

 Euceratherium. This is apparently not due entirely to the im- 

 maturity of the individual. The palatal portion of the maxillae 

 differs in the relative measurements. The anterior margin of the 

 posterior nares is between the crescents of the last superior 

 molars, Avhile in Euceratherium the margin is on a plane with 

 the posterior border of the last molars. The shallow fossae on 

 either side below the narial border in Euceratherium are absent 

 in Preptoceras, this region being more like that in Aplocerus. 



Dentition. — The dentition resembles even more closely that of 

 Ovibos than does Euceratherium, the length of the dental series 

 being the same, as that of an adult skull of O vibos in the Univer- 

 sity collection. 



