326 KANSAS UNIVERSITY SCIENCE BULLETIN. 



plesiosaurs is a secondary character, and whatever resemblance 

 there may be is merely a parallel one. The coracoid of the 

 plesiosaurs, which forms the broadest element in the girdle, 

 finds its homologue in the coracoid of the turtles which is 

 within the plastron and forms no part of it. The same may be 

 said of all of the elements of the pelvic girdle. The elements 

 which are supposed to form the plastron are the ventral 

 ribs, the interclavicle and clavicles. All of these structures 

 are present in the plesiosaurs, but they are also found in the 

 ichthyosaurs and nothosaurs. 



Williston has recently shown^'* that there can be no grounds 

 for relationship between the two groups on the structure of 

 the skull, and disclaims any relationship on the structure of 

 the vertebrae and paddles. The skull of the turtle is as far 

 removed in its structure from that of a plesiosaur as it well 

 can be. In the turtles the skull is completely roofed over, and 

 there is not a vestige of a temporal bar or vacuity. It is 

 known as the stegocrotaphous type of cranium. The plesio- 

 saurs, on the other hand, have the mammalian type of cranium, 

 and possess but a single bar and a single enclosed vacuity. 

 This type of skull Williston calls the therocrotaphous. It would 

 be incongruous to relate two groups of animals in which the 

 crania are of such widely varying types as are the skulls of 

 the turtles and plesiosaurs. 



There is a similarity in the mode of life of the two groups, 

 since both are largely of littoral habits. The similarity of 

 "form and habits of these two orders of animals has bean due 

 solely to parallel evolution, to similar aquatic conditions." 

 (Williston.) There is decidedly more similarity in the form 

 and mode of life of the ancient ichthyosaurs and the modern 

 dolphins, but no one has ever seriously considered relating 

 these two groups. 



The relationship between the turtles and plesiosaurs has 

 therefore been based, in large part, on misconceptions, and the 

 animals do not have the structur.es on which the greater claims 

 for affinity have rested. ''The plesiosaurs could not have been 

 derived from any ancestors that might by the widest stretch 

 of the imagination be called Chelonia or Chelonia-like. Nor 

 could the turtles have come from any forbears even suggesting 

 the sauropterygian structure" (Williston). 



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



