GREGORY: 



NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



221 



An alternative hypothesis is that the platyrrhine condition has been derived from a stage in which 

 the bulla had not yet overlapped the ring, as may also be the case in the Lorisidfe and Tarsiidse. Modern 

 lemurs including Chiromys actually pass through this stage in their development (F. Major, 1899, pp. 

 987-988). The possible objection to this hypothesis is that even although all lemuroids pass through 

 a stage in development in which the ring is outside the bulla, yet it is simply a primitive placental condition 

 antedating the proto-lemurine stage. 



Pari passu with the changes in the accessory auditory structures the position and course of the main 

 carotid artery and its stapedial branch were materially changed. It sank beneath the surface of the 

 periotic, lost the stapedial branch and passed through the body of the periotic, avoiding the basisphenoid 

 and emerging in the brain-cavity behind the sella turcica. This arrangement of the carotid and its 

 branches appears to represent an advance upon the conditions preserved in the Notharctina*, Adapinse, 

 Lemuridae, Indrisidse and Chiromyidse. If the arrangement of the carotid in the Platyrrhini is secondary 

 it is not unlikely that the auditory region itself is equally modified and that here as elsewhere in the brain 

 and braiu'-case the Lemuridse and, still more, their Eocene relatives the Notharctina', ha^'e retained the 

 ancestral conditions for all Primates. 



SUMMARY OF THE STRUCTURAL AND GENETIC RELATIONS OF XOTHAHCTrs WITH 



OTHER PRIMATES 



The skeletal remains of the Middle Eocene lemuroid Notharctus described above are of great interest 

 because they represent a primate which is at once the oldest and the most primitive that is known from 

 adequate material. They afford a knowledge of the skull, vertebrae, girdles, and limbs, and thus they 

 are of much greater morphological importance than many of the genera and species that were founded 

 only upon fragments of the dentition. They also serve to establish the geological antiquity of a general 

 skeletal type that is preserved, with some alterations, in the modern lemurs ; finally, by comparison with 

 other extinct and recent types, they supply an important chapter in the evolutionary history of the 

 entire order. Notharctus and its predecessors thus stand relatively near to the base of the order and 

 represent in many respects the earliest ancestors of the higher primates. They also tend to connect the 

 primates with some group of arboreal insectivores, probably the Mesozoic ancestors of the Menotyphla. 



The relatively primitive character of Notharctus becomes more evident when we compare it on the 

 one hand with other Primates and on the other hand with the Paleocene and Eocene mammals of many 

 orders, which are extensively represented in the collections of this Museum. 



The more striking osteological characters of Notharctus, as well as its remarkably primitive nature,, 

 may be summarized by contrasting it broadly with Hoyyio, Lemur and other Primates. 



Limbs and Vertebrae 



First, then, Notharctus was an arboreal, quadrumanous lemuroid of the Middle Eocene Epoch, whereas 

 Homo is a terrestrial bipedal, bimanous anthropoid of the late Tertiary, Quaternary, and Recent epochs. 

 This contrast in mode of locomotion, in environment, and ui geological age is reflected in the entire ana- 

 tomical difference between these respectively highest and lowest of the primates. The hands and feet of a 

 mammal more readily reveal its mode of locomotion and probable environment than does any other 

 part of the body, so that we look first to these elements in Notharctus, and we find that they differ only 



