THE MAMMALIA OF THE DEEP RIVER BEDS. 101 



the process, but allow the tip to project freely beyond them, which gives to the ven- 

 tral aspect of the odontoid a tri fid appearance. This feature is more emphasized in 

 the European form than in the American. 



Other cervical vertebrae accompany the specimen, but unfortunately they are so 

 badly damaged that little can be learned from them beyond the fact of their strongly 

 opisthocoelous centra and the generally equine nature of their processes. 



The posterior thoracic and lumbar vertebrae are likewise opisthocoelous and have 

 long, heavily built centra, with spines compressed and inclining forward ; the zyga- 

 pophyses are quite flat and show the equine character of cylindrical, interlocking 

 processes only in very moderate degree ; it is somewhat more distinctly displayed in 

 the lumbar region. 



Fore Limb (PL IT, Figs. 21, 22 ; PI. Ill, Figs. 2C-28; PL IV, Figs. 30, 31). The 

 humerus is of the same size as that of A. aurelianense, but differs from it in many 

 details of construction, in which it approaches the horse more closely than does that 

 species. As in the latter, the head projects much more strongly backward than in 

 the modern type, but resembles the structure of Equus more than that of A. aurelian- 

 ense in its greater flatness. The greatest difference, however, between the two species 

 of Ancliithe rium, in regard to the humerus, consists in the character of the tuberosi- 

 ties. According to Kowalevsky, the structure of the proximal end in the European 

 form is intermediate in character between the tapir and the horse; the external tuber- 

 osity is almost as large as in the former and the internal is also very similar to what 

 we find in that animal; but in the bicipital groove is a small, rounded eminence, the 

 beginning of the bicipital tubercle which reaches such prominence ia-Jilquus. In A. 

 equinum the tuberosities are more as in the latter genus ; the external one is much re- 

 duced, but the summit of the anterior portion rises higher than in the recent form, 

 while the crest to which the subspinatus muscle is attached has a greater antero-pos- 

 terior extent but is less elevated. The outer bicipital groove is much shallower than 

 in the horse and the bicipital tubercle, though broader, is much less prominent and 

 clearly defined. The external and bicipital tuberosities form a broad crest, which 

 rises much higher above the level of the head than in the existing genus and entirely 

 different from the corresponding structure in A. aurelianense. The shaft is massive, 

 broad, and flattened proximally, becoming round in the middle and flattening again 

 distally. The deltoid hook aud ridge are well developed, though less so than in the 

 horse, and the hook is placed higher up on the shaft than in the European species. 

 The distal end is much more equine in appearance than in the latter, though as in it 

 the trochlea projects more anteriorly than in the horse, which, in connection with the 

 stronger posterior projection of the head, gives to the whole bone a much more 



