442 Wortman — Studies of Eocene Mammalia in the 



and reduced ; its antero-external angle is produced into a strong 

 blade-like spur; the postero-external is wanting, and the inter- 

 nal is large. 



In the lower jaw, the incisors and canines are unknown. 

 The first premolar is small, single-rooted, and separated from 

 the canine in front and second premolar behind by diastemata. 

 The second premolar is two-rooted and also separated from the 

 tooth in advance and the one behind by considerable intervals. 

 The third and fourth are in contact. The first molar is the 

 smallest of the three, and not only differs from its fellows in 

 this respect, but the cusps of the trigon are less elevated. In 

 all of them the trigon is well developed, the shear is little 

 oblique, and the internal cusp is much reduced. The heels are 

 relatively small, with a high single posterior cusp and a faint 

 median ridge, that of the last molar being the largest. 



85 86 



Figures 85, 86. — Atlas and axis of Sinopa agilis Marsla ; natural size. 

 (Cotype.) 



While many other parts of the axial skeleton are preserved, 

 they are not sufficiently perfect to merit a very detailed 

 description. Of the series of cervical vertebrae, the atlas and 

 axis, figures 85, 86, and the body of the third or fourth are 

 present. The chief characters of these bones are the following : 

 The transverse process of the atlas is relatively large, extends 

 well behind the articular surfaces for the axis, and is thickened 

 and perforated at the base for the passage of the vertebral 

 artery ; the further course of the artery forward is similar to 

 that of the cat; the sinus atlantis is large, the cotyles deeply 

 cup-shaped, the superior arch broad from before backward, and 

 the inferior ring narrow and complete ; the axis has a large, 

 laminate, hatchet-shaped neural spine, which overhangs the 

 odontoid in front ; it is broken behind, but was presumably 

 well extended ; the odontoid is rather small and peg-like ; the 

 transverse spines are long, pointed, and pierced by the verte- 

 brarterial canal ; the ventral surface of the centrum has a very 

 strong median keel, which increases rapidly in depth poste- 



