A ik. 1903. North American Plesiosaurs — WlLLlSTON. 65 



Among the characters which have been given in the foregoing 

 descriptions there are some of more than usual importance, of more 

 than generic value. It is quite evident that the form can not be 

 placed in the same family with Dolichorhynchops or Cimeliasaurks 

 snowii. Just what family this may be I can not say at present. It is 

 very evident that we have to do in the plesiosaurs with several dis- 

 tinct families, but the material is hardly sufficient yet to clearly define 

 them. Several names have already been proposed, based upon partial 

 characters, but there is no unanimity in their acceptance, nor can 

 there be until much more is known about these animals than is the 

 case at the present time. 



The essential characters of the present genus, so far as known, 

 may be summarized as follows: Head large and broad; palatine 

 bones broadly contiguous; a strong pterygoid ridge on either side; a 

 deep interpterygoid fossa; neck very short; cervical ribs single- 

 headed; cervical ribs and vertebral arches united by persistent 

 suture; no infracentral vascular foramina. 



Many of these characters, possibly the union of the palatines in 

 the median line, are those of Plwsaurus; but Pliosaurus has the 

 anterior cervical ribs double-headed, a character supposed to be of 

 at least family, possibly subordinal value. Of this, however, I am 

 very skeptical, and it is possible that a final classification may locate 

 this genus with the Pliosauridae. 



It is very evident that the elongation of the neck is a specialized 

 character in the plesiosaurs, since we can not conceive of any animal 

 with so many vertebrae in the cervical region from which these animals 

 could be derived. Considering this character alone, Elasmosaurus 

 would be the most specialized of all the plesiosaurs, and Brae ha 11- 

 chetiius the most generalized. It is a question, however, whether such 

 forms as the present have preserved this primitive character from 

 their terrestrial ancestors, with only a slight increase in the number of 

 the cervical Vertebrae, or whether there has been a secondary reduc- 

 tion in the number from some long-necked ancestor. That the long- 

 necked plesiosaurs are not all specialized throughout, is very evident. 

 In the species of PUsiosaurus, a genus of long-necked forms, the 

 epipodial bones are far more generalized in character than are these 

 bones in the short-necked Polycotylus, where the epipodials have 

 become not only broader than long, but have actually increased in 

 number to four. That an increase of the number in the cervical 

 vertebrae is a specialized character has already been affirmed by Baur, 

 Dollo and Fiirbringer in the Dolichosaurs. 



It seems also evident that monocranial ribs are a specialization. 



