412 M. R. Thorpe — Leptauchenia Leidy and 



mm. 



Width above P- 24 



Width between middle of orbits 30 



Ant.-post. diam. of facial vacuity, approx 21 



Ant.-post. diam. of bulla 25 



Transverse diam. of bulla 19 



Depth of malar below middle of orbit 12.5 



Superior dental series with canine, length 51 



Superior molar series, length ! 25 



Superior premolar series, length 20 



Cyclopidius Cope 1878. 



The generic distinctions between Leptauchenia and 

 Cyclopidius are by no means clearly marked. Cope 

 defined Cyclopidius as being "Leptauchenia without 

 superior incisor teeth." As we now know the genns, 

 this is an inconstant character. Some of the species had 

 one or two superior incisors, and it is not unlikely that 

 all of them possessed two. These, however, may have 

 been, and probably were, in many cases very small and 

 perhaps vestigial, so that they were easily lost or de- 

 stroyed during the process of f ossilization. The inferior 

 incisors were likewise two in number. If we assume for 

 the present at least that Cyclopidius had but two incisors 

 in each tooth row, then we have the most marked generic 

 distinction between this genus and Leptauchenia, which 

 has the full complement of forty-four teeth. 



Cyclopidius is found in a higher horizon than Lep- 

 tauchenia, the latter being Upper Oligocene to Lower 

 Miocene and the former being restricted to the Middle 

 Miocene. 



In general, it seems that Cyclopidius possesses, in an 

 exaggerated form, the various peculiarities of Leptau- 

 chenia. The nasal vacuities are larger ; the frontals are 

 narrower and form but very little of the cranial roof; 

 the zygomata are much heavier and more widespread; 

 the cranium is very much narrower and smaller; the 

 nasals are narrow bands which expand slightly at their 

 junction with the premaxillaries ; the external styles of 

 the superior molars are more prominent; the inferior 

 caniniform premolar is somewhat reduced, and, in size, 

 more nearly approximates that of the true canine; the 

 external auditory meatus is somewhat more posteriorly 

 placed; the rami have become more robust and much 

 heavier posteriorly, and the whole skull foreshortened 

 and much more brachycephalic. 



