276 



TEETIAEf VEETEBEATA OF THE EATtfM. 



Turtles except Sphargis, the only living representative of the Athecse. The bone, in 

 fact, belongs to the most highly specialised type of swimming humerus, to which 

 Wieland * has given the name parathalassic. The whole bone is strongly compressed 

 dorsi-ventrally. The head (5.), so far as preserved, is strongly convex and somewhat 

 triangular in outline. The ulnar crest {a.) projects further beyond the head than 

 in Psephophorus scaldii or in Sphargis. Between the anterior thickening, which 

 terminates on the head, and the posterior border the surface of the shaft is concave 



Test-fig. 87 



Left humerus of Psephophorus eoccenus, type specimen : dorsal and ventral views. 

 a., ulnar crest ; b., head ; c, radial crest ; d., entocondyle. i nat. size. 



on the upper and lower faces of the bone, but the ventral concavity is deepest. The 

 part of the bone above the radial process is considerably more elongated in proportion 

 to its width than in Psephophorus scaldii and still more than in SpJiargis. The radial 

 prominence is very strongly developed, but it cannot be seen exactly what is the form 

 of its ventral surface, which, however, is not divided into two or more separate knobs, 

 as in Psephophorus, and is continued on the ventral face of the bone obliquely back- 

 wards, so that if the line of its direction were continued it would pass through the end 

 of the ulnar process ; in both Sphargis and Psephophorus the ridge is placed more 

 transversely. The dorsal surface of the shaft opposite the base of the radial process 

 bears a deep oval pit for the attachment of muscle ; in Sphargis this seems to be 

 represented by several smaller depressions. The distal end of the bone is incomplete, 

 but, like the upper end, it seems to have been less expanded than in Sphargis. There 

 is no trace of any ectepicondylar groove or foramen. 



On the whole, the bone resembles the humerus of Psephophorus rather than that of 



• Amer. Journ. Sci. [4] vol. ix. 1900, p. 420. 



