GREGORY: NOTHAECTUS, AN AMERICAN EOCENE PRIMATE 



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logons with the anteroinferior spine of the ihuni in man and served partly for the origin of the jiowerful 

 rectus femoris, a muscle of great importance in leaping. 



From the more extended comparisons made in the preceding pages it is inferred that Notharctus and 

 still more its primitive ancestors, the first primates, were somewhat less agile in their movements than 

 are their more highly specialized descendants ; that at first they climbed and leaped about the trees more 

 cautiously, clinging tightly to the branches, and " brachiating " or leaping with the arms, less effectively. 



Skull 



When we compare the skull of Notharctus with those of Homo and of the higher primates the difference 

 in general appearance is so great that were it not for the existence of a number of structural intermediates 

 the cautious morphologist would hesitate to affirm that the remote ancestors of man had any special 

 resemblances in the skull to this humble lemuroid. 



In man, and to a less extent in the young of the great apes, the brain-case, especially the chamber 

 that lodges the cerebrum and the cerebellum, is enormously expanded and dome-like; the facial part 

 of the skull, which lodges the olfactory organs and supports the dentition, is excessively shortened and 

 retracted beneath the anterior end of the brain-case. In N^otharctus on the other hand, as in many other 

 primitive mammals, the brain-case forms only about half the total length of the skull; the face is elongate 

 and fox-like, with large olfactory chamber and long jaws, and is situated wholly in front of and not below 

 the brain-case. 



In man the premolars are reduced to two on each side above and below, in correlation with the short- 

 ening of the face; Notharctus on the other hand retains four premolars on each side above and below as 

 do also the most primitive placental mammals. 



In man the opposite premaxillary bones are much retracted and are fused with the true maxillae; 

 in Notharctus on the contrary the premaxillaries remain separate, occupy their primitive mammalian 

 position and are extended far in front of the maxillaries. 



In man the forward growth of the temporal lobes of the brain has, as it were, pushed forward the 

 ascending wing of the alisphenoids so that they have gained contact with the orl)ital wall of the malars 

 and with the postorbital flanges of the frontals, these three elements thus closing the orbital cavity 

 posteriorly. Essentially identical conditions obtain in the great apes and in all the Old World and New 

 World monkeys, but in Notharctus, as w^ell as in "the existing lemurs, this closing has barely begun: the 

 temporal lobes of the brain and the temporal wings of the alisphenoids still lie far behind the orbits, 

 which therefore open widely into the temporal fossa?. The chief feature which distinguishes Notharctus 

 as a primate in this region is the narrow union of the ascending orbital process of the malar with the 

 descending postorbital process of the frontal, so that the orbit is guarded posteroexternally by a bony 

 ring as in lemurs. In man the orbits are directed forward; in the far more primitive Notharctus the 

 orbits look outward and upward as well as forward. 



In man as well as in the young of all the great apes the top of the brain-case is not surmounted by 

 a median or sagittal crest for the attachment of the temporal muscles, and the dorsal limit of these muscles 

 does not extend to the mid-line ; but in Notharctus the occiput is narrow and not expanded and the lamb- 

 doidal crests which form its margins are sharp j^rojecting ridges; so that in this region, as well as in the 

 whole lateral and superior aspects of the skull, Notharctus is like an opossum, one of the most primitive 

 of existing mammals, and its skull thus seems at first sight totally different from the large-brained, short- 

 faced skull of man. 



