110 



GREGORY: XfyrilARCTUS. AN AMERICAN EOCENE PRIMATE 



size of the cotylus for the occipital condyle, its flatness and more anteroposterior direction. (2) The 

 stoutness and greater expansion anteroposteriorly of the transverse process. (3) The foramina and canals 

 for the vertebral artery and for the suboccipital nerve are smaller. (4) The transverse process does not 

 bridge over anteriorly the common canal for the vertebral artery and the suboccipital nerve, whereas in 

 Lemur it does bridge over this canal and is thus joined to the anterior rim of the atlas. Another speci- 

 men of L. mongoz, No. 22886 (Fig. 26.2), is far more slender and the openings for the vertebral artery 

 and suboccipital nerves are much expanded, while the neural arch is very narrow. Thus the difference 

 between these two atlases of Lemur mongoz is greater than the difference between one of them and that 

 of Notharctus. 



The atlas of Notharctus in general is not dissimilar to that oi Propitliecus, except for the wide openings 

 in the latter for the ner\'es and arteries. 



As compared with the atlas of Cebus, that of Notharctus is stouter in all its parts; the facet for the 

 occipital condyle is smaller and the openings for the nerve and artery are far more restricted; also the 

 canal for the spinal cord was probably much smaller. As compared with the atlas of the macaciue, similar 

 differences obtain; the latter is of more slender structure and the openings for the spinal cord and for 

 the vertebral artery and suboccipital nerve are much more expanded ; the transverse process is also more 

 delicate and pointed and the hypapophysis is extended ventrally, this implying a stout longus colli muscle. 



Figures 26.1-5 show the course of the vertebral artery in these primates. As in other mammals it 

 pierces the transverse process posteriorly, then turns sharply inward and joins the canal for the sub- 

 occipital nerve, again turning sharply forward and running into the brain-case through the foramen 

 magnum. In Notharctus the smaller size of the openings for the spinal cord, for the suboccipital nerve, 

 and for the \'ertebral artery is in harmony with its smaller brain and probably smaller blood-vessels; 

 the existing forms, Lemur, Cebus, Macacus and the higher primates, showing a progressive enlargement 

 of these elements. 



Axis (PI. XXXII). — The base of the axis is preserved in No. 11473, referred to Notharctus osborni. 

 It agrees so closely with that of Lemur mongoz that in all probability the neural arch and neural spine 

 displayed an etiual resemblance. The lemur t^^pe of axis contrasts sharply with that of the creodont or 

 of the modern carnivore in being much shorter anteroposteriorly, with smaller odontoid and with the 

 neural arch and spine shorter and less expanded; the transverse process also is less expanded. These 

 characters are correlated with the more slender occipital and cervical muscles of the lemurs, in contrast 

 with the powerful neck of the carnivores. 



The axis of Notharctus differs from that of Lemur jnongoz in the more delicate odontoid process, in 

 the less pronounced median hypapophysial ridge, and in the lack of a median posterior process. Lemur 

 varius has all these characters more pronounced and progressive. The transverse processes in Lemur 

 point l^ackward more sharply than do those of Notharctus. 



Cebus has the base of the axis widely produced posteriorly in the median line, in correlation with 

 the more sharply curved neck, and the transverse processes are much larger and more divergent; the 

 neural arch is more slender and the odontoid heavier, all these characters being associated with the ex- 

 panded occiput and with the more upright pose of the animal. (Plate XXXTI.) 



Macacus (PI. XXXII) has the odontoid larger and more protuberant; the transverse processes, as 

 seen from below, form a pair of large sharply pointed wings, and the median ventral portion is widely 

 produced posteriorly in correlation with the marked curve of the neck and upright pose of the head ; the 

 neural arch is very high and the neural opening very large; the posterior zygapoi)hvsial facets are more 



