GREGORY: XOTHARCrVS, AN AMERICAN EOCENE PRIMATE 



237 



and grasping is complicated by more restless and varied movements: running on the branches, brachiating, 

 sitting upright, using the hands to manipulate the food, etc. In some of the Cebidse the hands become 

 hook-like, the pollex being lost. In the nearly allied Hapalidse the nails become compressed into claws, 

 which resemble those of tree shrews. In correlation with these differences the metapodials are longer, 

 the phalanges shorter, the ungual phalanges compressed, the fourth digit is no longer than the third. 

 The great process on the proximal end of the hallux for the tendon of the peroneus longus muscle is less 

 developed. That these conditions are all secondary, and not as primitive as the conditions seen in Noth- 

 arctus, is suggested by the fact that the Cebidae have obviously evolved in the direction of the higher 

 primates, as fully shown by the advanced stage of construction in the auditory region, brain-case and 

 dentition. 



In conclusion, the chief stages in the evolution of the limbs of Primates appear to be as follows: 



(1) Permian. Primitive therapsid or gorgonopsian stage, of which the hands and feet are well rep- 

 resented by Scymnognathus (Broom, 1913) and Theriodesmus (Seeley, 1888). The phalangeal formula is 

 transitional from the primitive reptilian (2, 3, 4,5, 3) to the mammalian formula (2,3,3, 3,3). Of the 

 primitive elements of the reptilian shoulder girdle only the cleithrum is lacking; coracoid and metacora- 

 coid both present. The elbows and knees everted less than in primitive crawling reptiles. Humerus 

 very primitive, with flattened head and widely spread l^icipital fossa. Femur with flattened sessile head and 

 no neck. Great trochanter a low ridge on side of shaft. Ilium directed forward. Pubi-ischium plate-like. 



(2) Triassic. Cynodont reptilian stage with very short spreading pentadactylate feet. This 

 stage is known in Microgomphodon and J^Jlurosuchus. The carpals and tarsals are plainly homologous 

 with those of mammals. A second centrale carpi, which has been lost in the mammals, is retained. The 

 phalangeal formula is 2, 3, 3, 3, 3, as in mammals. Phalanges and metapodials very short. Scapula 

 with everted anterior border forming the spina scapulte. Prespinous fossa at most incipient. Pectoral 

 girdle as a whole approaching the monotreme stage. Hiunerus submammalian but with flattened head. 

 Femur with large great trochanter and a small lesser trochanter. Head oval, sessile. Ilium directed for- 

 ward with expanded gluteal surface. A small obturator fenestra. Epipubic bones probably present. 

 Elbows more or less everted, knees straightened in running. 



(3) Jurassic (?). (Predicated from indirect anatomical and palseontological evidence.) Primi- 

 tive arboreal metatherian stage. Hallux and pollex divergent. Digits moderately short, with compressed 

 claws. Pads and friction ridges specialized to secure firm grasp on branches. Tarsus very short, astrag- 

 alus practically without neck. Coracosternal connection broken, coracoid short, only a single coracoid 

 present. Scapula with prespinous blade. Humerus with convex head and long delto-pectoral crest. 

 Femur with ball-like head and distinct great trochanter. Digital fossa pronounced. Ilimn long and 

 narrow. Thyroid fenestra enlarged. Epipubic bones present. Elbows and knees drawn in towards 

 flanks. Moderate supination of forearms and inturning of hind feet. Such animals doubtless climbed 

 slowly and clung to the branches. 



This primitive metatherian stage is largely preserved in the Cretaceous and modern opossums. 

 It gave rise at a later epoch to all the specialized marsupial types and possibly also to the stem placental 

 (stage 4) . 



(4) Cretaceous (?). (Predicated from indirect anatomical and palseontological evidence.) Primi- 

 tive arboreal placental stage. Differs from (3) chiefly in the improvement of the tibio-tarsal joint through 

 the development of a cUstinct neck on the astragalus, and the wedging upward of the astragalus between 

 the fibula and the tibia, so that the articular surface for the fibula comes to lie nearly at right angles with 



