EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEEES 



299 



of the original forms. They exhibit many features in 

 common, yet they seem to represent two distinct 

 contemporaneous phyla. Of these two phyla Lim- 

 nohyops is the more primitive; it is in many features 

 more central or intermediate than the second phylum, 



ferent lines of descent, the ancestral members of 

 each line (Bridger A and B) are not easily distin- 

 guished. 



The abundance of remains of Palaeosyops is wel- 

 comed by the student of evolution because so many 



Figure 254. — Sections of skull of brachycephalic (A, B) and dolichocephalic (C, D) Eocene titanotheres 



One-fourth natural size. Ai, Palaeosyops Icidyi, Am. Mus. 1544 (type). Median section of cranial region. Note the back part of the 

 olfactory chamber (with remnants of the ethmoturbinals), the cribriform plate, the frontal sinus, and the cellular character of the expanded 

 cranial vertex above the brain chamber. (Section line shown in A2.) As, The same, rear view. The left side shows the widened vertex oi the 

 occiput; the right side shows in section the frontal and ethmoidal sinuses. B, Limnohyops prisms, Princeton Mus. 10044. Cross section 

 through the basioccipital, periotic, squamosal, and paiietal, seen from front. In this primitive form the sagittal crest has not expanded 

 into a flattened cranial vertex, and hence this region is without any large cavities. C, Dolichorhinus Jiyognathus, Am. Mus. 1851. Median 

 section of the whole skull slightly to the right of the median plane, showing the enormous olfactory chamber, the elongate fronto-occipital 

 sinus, and the small brain chamber. The much enlarged maxilloturbinal (mx. tb.) is produced backward and downward, appearing as a 

 prominent swelling in the roof of the narial channel; the primary border of the posterior nares is at pn', the secondary at pn'. D, Dolichorhinus 

 longicepsf (Jiyognathus f) , Am. Mus. 1852. Cross section through the middle part of the brain chamber (near line A-A of figure C) looking 

 forward. Note the fossae for the anterior lobes of the brain, the lateral ethmoid sinus, the mesethmoid septum, the remains of the 

 ethmoturbinal scrolls, and the large fronto-occipital sinus. 



consisting of the very massive, broad-headeu Palaeo- 

 syops. 



The genus Palaeosyops of Leidy was the first known, 

 and the LimnoJiyops of Marsh may be regarded as a 

 subgenus. Although the animals belong to two dif- 



mtergradations or mutations are found. But this 

 very abundance renders more difficult the definition of 

 species because the sharp lines of specific separation 

 and distinction breah down ; the forms merge into one 

 another. 



