Apr. 1903. NORTH AMERICAN PLESIOSAURS—WILLISTON. 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 Dodichorhynchops or Cimoliasaurus 
snow. 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 Plosaurus; 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 Phosauride. 
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 vertebre in the cervical region from which these animals 
could be derived. Considering this character alone, //asmosaurus 
would be the most specialized of all the plesiosaurs, and Arachau- 
chenius the most generalized. It isa 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 vertebre, 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 Plestosaurus, a genus of long-necked forms, the 
epipodial bones are far more generalized in character than are these 
bones in the short-necked /Po/ycotylus, 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 
vertebre 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, 
