MOODIE: TURTLES AND PLESIOSAURS. 325 



of the scapula of turtles and plesiosaurs. In the turtles the 

 element is fundamentally biradiate, but in the plesiosaurs it is 

 merely a curved plate-like element, which hardly has the ap- 

 pearance of the V, so characteristic of the Chelonia. The radi- 

 ate character of the scapula of plesiosaurs is ontogenetic, as 

 Andrews has shown.^^ The scapula of turtles early assumes 

 the V shape, and this is evident in very young embryos. 

 The radiate character of the plesiosaurian scapula is not at- 

 tained until the animal becomes adult. The three rays thus 

 formed by the growth of the outer process was used by Hulke 

 for correlation with the shoulder girdle of the turtles. But 

 this correlation falls to the ground when we learn that the 

 third ray of the chelonian girdle corresponds morphologically 

 with the broad pectoral element, the coracoid, of the plesio- 

 saurs. It has been shown above that the turtle scapula is a 

 single element, and the singleness of the plesiosaurian scapula 

 is vouched for by Seeley, Baur and Williston. "As to the 

 structure of the scapula, all students of the plesiosaurs are 

 now agreed that the procoracoid does not unite with the scap- 

 ula, . . ."^- The scapulae of turtles and plesiosaurs are 

 analogous in their form but they differ greatly and funda- 

 mentally in the two groups by the formation of the third ray 

 in plesiosaurs. Seeley^-^ has agreed with the results above 

 given. He likewise was of the opinion that Hulke had misin- 

 terpreted Rathke. Hulke's greatest error lay in giving a por- 

 tion only of Rathke's results and taking that portion as positive 

 evidence for two ossific centers. Seeley objects strongly to the 

 idea that there is any procoracoid in the scapula of the plesio- 

 saurs and disclaims any relationship between turtles and ple- 

 siosaurs on the basis of the structure of the shoulder girdle. 



The whole discussion of the relationship of turtles and 

 plesiosaurs, as based on the elements of the pectoral girdle and 

 the "epiphyses," has revolved around structures which are not 

 present in either group. Neither group has epiphyses, and in 

 neither group is the scapula composite. There is thus no basis 

 for an affinity on the ground of these structures. 



The claim that the broad pectoral and pelvic elements of 

 the plesiosaurs are homologous to the plastron of turtles can 

 hardly be valid. The broad character of the girdles in the 



11. Andrews, 1895, Ann. and Mag. Nat. Hist., ser. 6, vol. XV. 



12. Williston, 1907, Proc. U. S. Nat. Mus., vol. 32, p. 488. 



13. Seeley, 1893, Proc. Roy. Soc. Lond., p. 161. 



a-Univ. ScL BulL, Vol. IV, No. 15. 



