NO. 1540. THE SKULL OF BRA CHA UCHENIVS— WILLISTON. 485 



Relationships of Brachauchenius. — The most distinctive characters 

 of the genus are found in the broadly united palatines, the broad 

 union of the pterygoids posteriorly, the short, deep-set interptery- 

 goidal vacuities, the ridge-like buttresses of the pterygoids, the 

 remarkably small number of the cervical vertebrae, the absence of 

 vascular foramina on their under side, the single-head cervical 

 ribs, etc. The dorsal surface of the skull has a remarkable resem- 

 blance to that of Pliosaurus ferox, as figured by Andrews. Andrews 

 assumed that the palatines in his specimen were separated by the 

 pterygoids throughout, but expressl}'^ says that indications of the pala- 

 tine relations posteriorly are wanting. I have scarcely a doubt but 

 that they will be found to have the same structure as in Brachau- 

 chenius in better preserved specimens of the genus. 



Much stress has been placed upon the palatal structure in the 

 reptiles as indications of phylogenetic relationships, but I have never 

 had a great deal of faith in the stability of these parts. Here we have 

 the union or separation of the palatines in thcr same order. The 

 general shape of the skull, the depressed parietals, and, I am confi- 

 dent, the relations of all the other bones of the upper side of the skull, 

 are all nearly alike in Pliosaurus and Brachauchenius. Furthermore, 

 in the reduced number of the cervical vertebra^ in the two forms, 18 

 or 20 in the older, 13 in the younger, we have a genetic resemblance, 

 I believe, one that strengthens my assumption that the shortened 

 neck in the later forms is not a primitive character, but a degenerate 

 one, one that has been acquired in more thai one phylum. Indeed, 

 so far as all these characters of the skull go, in the probability that the 

 arrangement of the skull bones will be found essentially alike in the 

 two genera, I should hesitate to separate the two types generically, 

 were it not for the cervical ribs, single-headed in Brachauchenius, 

 double-headed in Pliosaurus. The character of the cervical ribs has 

 been considered as of more than generic importance, Seeley even 

 proposing an ordinal subdivision based upon the divided or undivided 

 neck-ribs. Here, too, I believe that the fusion of the imperfectly 

 differentiated heads is a feature common to more than one line of 

 descent, and is of no more than generic importance. It is a fact that 

 all known American Cretaceous plesiosaurs have cervical ribs with 

 undivided heads, and that is probably the case with all Cretaceous 

 forms, as it is also with the known American Jurassic ones. Double- 

 headed ribs are a primitive character confined to the early forms, for 

 the most part. 



In conclusion, I would suggest that the family Pliosaurida^ be 

 maintained, based upon the common characters apparent or probable 

 in Pliosaurus and Brachauchenius. 



The characters of Brachauchenius, so far as they are now known, I 

 give as follows: 



Brachauchenius. — Mesocephalic. Teeth not more than 2U in each 



