76 



GREGORY: NOrilARCTUS, AN AMERICAN EOCENE PRIMATE 



(12) The head is larger and somewhat more spherical. 



(13) The greater and lesser tuberosities are nearer together and the bicipital groove is narrower at top. 

 Thus the humerus of Alouatta may be regarded as in many respects a degenerate and de-differentiated 



derivative of that of Notharctus. These characters in general indicate more ape-like ways of using the 

 arms, fuller extension, freer movements at the shoulder, stronger and more frequent pronation of the 

 forearm, greater strength in flexing the carpus. 



In Cebus and Chrysothrix the humerus is less highly specialized, since a well-developed entepicondylar 

 foramen, an outer lip of the trochlea, and a better developed supinator crest are retained; but here a 

 characteristic ape-like specialization is the gentle bending of the upper part of the shaft toward the inner 

 border of the humerus. In the Hapalidse the deltoid plane is flattened and more or less V-shaped and 

 the humerus as a whole could easily be derived from the N' otharctus type, but close affinity with the Cebus 

 type is evident. 



Hence it is quite obvious that with regard to the form of the humerus the Platyrrhini are not as 

 near to Notharctus as are the lemuriform genera, especially Adapts, Lepilemur, Lemur, Chiromys, and 

 Propithecus. 



The catarrhine types of hiunerus (inclutling those of Old World monkeys, apes, and man) have 

 already been compared with the Notharctus type (p. 64 above). They show rather close resemblances 

 to the platyi'rhine types, but usually have a better developed trochlea with prominent inner and outer 

 lips. Thei-e is no trace of the entepicondylar foramen and the supinator crest is inconspicuous. The 

 deltoid plane is accented and so is the bicipital groo\-e. The head is more or less spherical. Brachiat- 

 ing ^ and the habit of sitting upright, culminating in the anthropf)ids, are chiefly responsible for these 

 differences. (Plate XXVH; Fig. 7.7, Cehus, Pan.) 



This series of comparisons makes clear the morphological importance of the very primitive primate 

 humerus of Notharctus, which, while retaining many primitive mammalian characters, is well differentiated 

 from the humeral types of marsupials, creodonts, carnivores, taligrades, edentates, rodents, insectivores 

 and others. Its nearest structural ally outside of the primates is Nothodectes, which is now referred to 

 the Menotyphla. Within the order Primates the humerus of Notharctus again is extremely primitive, 

 and fitted to give rise in turn to the humeral types of the Lemuriformes, Lorisiformes, Tarsiiformes, 

 Platyrrhini and Catarrhini as understood by the writer. But the nearest structural resemblances are 

 to be found among the Lemuriformes, especially among the Adapidse and Lemuridae. Hence the humerus 

 of Notharctus, as well as the great majority of all other elements of the skeleton, is plainly in a lemuriform, 

 or better, in a pre-lemuriform stage of evolution. 



The humeral type of the Adapinse is very close indeed to that of Notharctus. 



The humeral types of the Cebidae have departed more widely from the primitive Notharctus type: 

 in the greater elongation and cylindrical form of the bone, in the marked reduction of the external epi- 

 condylar ridge, delto-pectoral crest, and entocondyle; the entepicondylar foramen is often absent; the 

 trochlea and capitellum now form one widely extended, almost cylindrical joint. These details of the 

 humerus of the Cebidae, taken in connection with the characters of the manus, indicate that in general 

 these animals do not have so tight a grip upon the branches but that this inferiority is more than com- 

 pensated by a superior quickness and agility. Here, as in many other lines of primates, the progressive 

 advance in brain capacity has been associated with progressive adaptations for versatility and agility 

 at the expense of robustness and passive clinging power. 



I Profrrcssiiif!; I>y swinsiufi' frcini hrancli t.. hnincli with the foivanns. (Compare Keith, 1899, Prof. Zool. Soc, Mar. 7, p. 305.) 



