TRANSACTIONS OF WAGNER 

 Il8 



UINTA SELENODONTS 



molar ; there is no masseter crest and the masseter surface is almost obsolete. 

 The nasal canal is high and roofed over by short nasals ; the posterior nares 

 are also high, especially in the pterygoid portion. Characteristic in the 

 cranium is the small extent of the frontal zone as compared with the parietal 

 zone, in the formation of which the very high squamosals take a large share. 

 The orbits are very prominent. A consequence of these relations is the dis- 

 placement of the orbits into the facial region above the molar alveoli, and the 

 extraordinary length of the temporal fossa. Sagittal and occipital crests are 

 prominent. The glenoid cavity is small, but has a high postglenoid process, 

 and very generally the angle of the mandible is produced behind the condyle. 



The applicability of this description is least obvious in the case of Proto- 

 cetas, in which genus the skull is much modified, approximating it in some 

 respects to the higher Pecora, the Cavicornia. This is to be seen in the edentu- 

 lous premaxillaries (though the incisive foramina remain quite small), in the 

 shifting of the orbit behind the molar alveoli, and in the downward bending of 

 the face upon the cranial axis, the latter not recurring in any other member 

 of the Tylopoda. Further, the angle of the lower jaw is not produced into a 

 hook-like process behind the condyle. In spite of these deviations, the essen- 

 tial features even of this skull are manifestly in agreement with those of the 

 other Tylopoda. 



At the other extreme of the scale stands the skull structure of the Oreo- 

 dontidce and Agrioclicerida, but in these cases also the definition will apply. 

 These families both retained throughout their history a very primitive type of 

 skull, which gives quite a close and deceptive resemblance to that seen in the 

 anoplotheres and anthracotheres. 



In two respects the skull in the main phylum of the Tylopoda differs 

 from that of all the side branches ; namely, in the form of the mandibular 

 condyle and in the cancellous structure of the tympanic bulla. The history 

 of this phylum shows, however, that both of these peculiarities were acquired 

 after the family Cauiclidce had begun its separate existence. Even in Poe- 

 brotherium the condyle is still transverse and displays little tendency to assume 

 the spheroidal shape, while in the earlier genus Protylopus the auditory bulla 

 is small and free from cancellous bone. Were the cervical vertebrae of the 

 latter genus known, we should probably find that the extraordinary peculiarities 

 of these vertebra; were likewise acquired within the limits of the family. 



The study of the Uinta selenodonts teaches us very forcibly the im- 

 portance of giving due weight to zoogeographical considerations in dealing 



