436 Bidlfti)/ American Museum nf Natural History. [Vol. XXVII, 



out-curved rod with a knob at the top {Micrugalc) to a spatuhite form with 

 rounded upper border (Tupaia). 



The humerus. 



The series of humeri figured herewith (Fig. 27) brings out many inter- 

 esting facts, especially the following: The humerus of Echidna is seen to be a 

 modified form of that foreshadowed in the Anomodontia, as represented 

 (Fig. 27, no. 1) by the American Triassic genus Euhrackiosaurus Williston. 

 Noteworthy features are the very broad entocondyle, the large greater and 

 lesser tuberosities, the globular capitellum, and relatively weak ectocondyle; 

 the ulna apparently took no part in the front face of the trochlea. The 

 Marsupial type (Fig. 27, nos. (i-lO) is directly derivable fi'om the type seen 

 in the Cynodont Gomphognaihus {cf. p. 119). The Marsupials seem to be 

 characterized by the prominence of the sui)inator crest which in many primi- 

 tive Placentals {e. g., Fig. 27, nos. 11-18) is less developed. The entepi- 

 condylar foramen has its primitive elongate character in several Marsupials, 

 Edentates, Periptyckus, etc. Very noteworthy is the loss of the entepi- 

 condylar foramen in Castor, Arcto7ni/s, and Erinaceus, as well as in all but 

 the most generalized ungulates. 



The primitive form of the humeral capitellum appears to be globular. 

 In the ambulatory and cursorial forms (Fig. 27, nos. 7, 12, 16, 18) it be- 

 comes transversely cylindrical. The internal part of the trochlea was perhaps 

 at first not very wide and served only for the radius {cf. p. 433). The hu- 

 meri of Myrmecophaga and the Sloths are very peculiar (Nos. 3, 5). The very 

 wide entocondyle and entepicondylar foramen, joined with the globular capi- 

 tellum, may ])ossibly be primitive features. Analogies with the Mono- 

 tremes on the one hand and with Castor on the other are evident. 



The series furnishes an instructive example of the manner in which 

 palfEotelic and cjienotelic features are mingled in the humerus of every tyj)e. 

 When classified in accordance with the supposed habits of the animals the 

 humeri would be groupetl about as follows : 



Arboreal. ■ Semi-fossorial. Ambulatonj. Cursorial. 



jDidelphis (6). f Echidna (2). f Arctocyon ( 1 1 ) . f Viverravus (12). 



[ Phalanger (10). [ Eubrachiosaurus (1). j Solenodon(15). \ Euprotogonia (18). 



? Hapalops (5). Myrmecophaga (3). [ Periptychus (17). Thylacynus(7). 



Dasyjaus (4). f Arctomys (14). 



Phascolomys (8). [Erinaceus (16). 



Castor (13). 



The brackets indicate close resemblances. 



