GREGORY: NOTTIAHf'Tl'S, AN AMERICAN EOCENE PRIMATE 



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(2) The posteroexternal limit of tlie deltoid plane is a prominent ridge, \\ hile in Noiharchis no sucli 



ridge is present. 



(3) The capitellum is large antl truncate externally and usually there is not even the I)eginning of 



an external lip on the trochlea. 



(4) The shaft of the humerus below the deltoid eminence is sharply triaiigulai- while in Xollidrdus 



it is rounder in front. 



(5) The entocondylar process is nmch larger, this indicating great strength and size of the i)rona- 



tor and flexor muscles, as is usually the case in mammals with stout claws. 



In all these characters in which Notharctus differs from creodonts it agrees with existing lemurs. 

 These characters are all connected with advanced arboreal habits in contrast with ihc terrestrial habits 

 and unguiculate structvu'e of the most primitive creodonts. 



Among recent carnivores, the arboreal Procyonida^ show the nearest approach to Nolluirclu.s in the 

 general form of the humerus. They differ, howe\er, in many details, especially: 



(1) The deltoid crest is nmch larger and has a spirally wari^ed outer i)lane, recalling that of other 



carnivores. 



(2) The entocondyle is much lai'ger, this proljably implying stouter flexor and pronator nuiscles 

 as in animals with stout claws. 



(3) The capitellum is less ball-hke, this implying less developed power of supinating the i-adius. 

 In brief, the Procyonida? may very w(41 be secondarily ai't)i)real d(M•i\'ati^'es of a Ci/tioilicHs-Wke type, 



while Ahtharctus shows a deeper and more long continued impress of arboi'cal liabits. 



Taligrada. The humeri of the Paleocene Taligrada £/>/oro////8 -dnd Fan loldinhda hiiw all the primi- 

 tive characters listed above in connnon with the humerus of Xolharclns. All their crests and process(\s, 

 however, are wider and more robust. Their triceps, brachialis anticus, supinator longus, and other 

 arm muscles were of great width and strength. The great size and robustness of the entocondylar process 

 is only exceeded by Orijcteropus and Taxidea, which have very powerful pronator and flexor muscles in 

 connection with earth-scratching habits. The forearm was normally bent approximately at a right angle 

 to the humerus and the degree of maximum extension of the forearm was very limited. The forearm 

 could not be completely supinated. They nmst have pushed their way through a very resistant medium 

 of some sort, possibly the matted undergrowth of forests (Herbert Lang). Their humerus resembles 

 that of the ground-liA'ing Solenodon rather than that of the fully arboreal Xotliarctus. (Plate XXVII.) 



Edentata. Probably the most prunitive known edentate is Pahvanodon ignavus Alatthew (191S) 

 from the Lower Eocene. The short, wide humerus of this animal, Avhile sharing many primiti\'e cliar- 

 acters, contrasts widely with that of Notharctus in its ath'aneed fossorial adaptations. The head of 

 the humerus is an elongate (nal, directed backward and permitting free anteroposterior mo\'ements. 

 The delto-pectoral crest is much enlarged, flattened and somewhat V-shaped below. The entocondylar 

 process is larger. This is fairly close to the modern armadillo type. At the other extreme of the eden- 

 tates, the sloths exhibit a somewhat primate-like humerus with an extremely long shaft and degenerate 

 crests and processes but with some cmiously re\'ersed characters, associated witli the peculiar mode of 

 progression. 



RoDENTiA. The humerus of the x'ery primiti\'e Eocene rodent Parann/s is much nearer to that 

 of Notharctus than any of those described above and the muscle insertions must liave been very similar 

 (Plate XXVH). The chief difterences are as follows: 



(1) In Paramijs the delto-pectoral crest is much higher, ending below in a i)rominent eminence. 



