108 KANSAS UNIVERSITY QUARTERLY. 
from the one previously published only in the less flattened skull, 
and in the curvature of the digits. 
Platecarpus is based chiefly upon one specimen, comprising a 
nearly complete disarticulated skull and a connected series of ver- 
tebra to beyond the middle of the tail, the sixty-fifth, together 
with the pectoral and pelvic girdles and most of the bones of the 
limbs. The arrangement of these bones has been copied, from 
Marsh, with some changes. The only parts conjectural are the num- 
ber of long ribs and the number of chevron-caudal vertebrae. Iso- 
lated bones and partly connected series of caudal vertebra are 
preserved in other specimens, from which there seems to be very 
slight differences from the corresponding parts of Zydosaurus. The 
tail has, therefore, been made to correspond with that of Zy/osau- 
rus in length. 
Tylosaurus is drawn from three specimens, one with the posterior 
part of the head and the vertebral columa complete to the tip, 
the second with the skull and cervical vertebre in perfect preser- 
vation, the third with the paddles nearly complete, together with 
the larger part of the vertebrae and ribs. This last specimen is 
the one of which figures of the paddles and skin were given by me 
in the last number of this journal. All of these specimens agree 
closely in size and characters, clearly belonging to the same spe- 
cies, 
A comparison of these genera, as shown by the restorations, will 
be of interest. Platecarpus has an intermediate position between 
Clidastes and Tylosaurus, which represent the extremes of develop- 
ment of the Kansas forms. In C/idas/es the thorax is elongate, 
the tail relatively short and modified into a powerful propelling 
organ. The limbs are small, the hind ones especially so. The 
animal throughout is more slender, and the head relatively short,’ 
agreeing in this respect more closely with their nearest modern re- 
latives, the species of Varanus. The vertebra have the firmest and 
closest articulations, with the interlocking zygosphene best devel- 
oped of any of the Mosasaurs. The limbs are less flexible, but re- 
latively stronger, as shown by the closely articulating bones and 
the fully developed carpus and tarsus, and the more pronounced 
processes for muscular attachment. The movement through the 
water in this form was more snake-like than in the others, and pro- 
pulsion was largely by means of the tail. 
In Platecarpus we have the same shortened muzzle as in Clidastes, 
the vertebre also relatively slender, and the zygosphene imper- 
fectly developed. The paddles are more of the Cédastes type than 
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