Williston.] Mosasaurs, 159 



paddle almost absolutely complete and all the bones, or nearly 

 all, in their natural relations. A photograph of the specimen is 

 shown in plate xlyiii, and in the accompanying figure is given 

 the outline of the bones there concealed, as determined from 

 an excavation on the opposite side of the slab. The number of 

 phalanges were apparently as follows : I, 6 ; II, 9 ; III, 10 ; IV, 

 11 ; V, 11. The distal one is preserved only in the fifth finger, 

 and is, as is seen, very small and imperfect. I am rather in- 

 clined to the opinion that the number of phalanges is not 

 always absolutely uniform in different individuals, though 

 probably varying only within small limits. It will be observed 

 that the fifth finger is longer by far than in either Platecarpus 

 or Clidastes. It is this specimen in which the remains of the 

 skin are preserved, described in the chapter on the restoration. 

 Between the phalanges, even to their extremities, traces of the 

 skin are found, from which it is evident that the membrane 

 connecting them was very thin and pliable and extended fully 

 to their tips. Small, scale-like scutes are found as far as the 

 metacarpals, beyond which they are wanting everywhere. 



In the plates cited above will be seen what are evidently the 

 natural positions of the digits of the front paddles in Plate- 

 carpus and Tylosaurus. The restorations of the paddles given by 

 Marsh are certainly unnatural and unlifelike. There is a gentle 

 curvature of the fingers away from the radial side. While in 

 life the fingers were probably less approximated, it is certain 

 that the fingers were never spread, fan-like. The first finger 

 appears to have been closely approximated to the second, and 

 incapable of much divarication, giving support and strength to 

 the side of the paddle. In the structure of the paddles espe- 

 cially, Tylosaurus is the most specialized of the Mosasaurs, and 

 the least lizard-like. The paddles are the most slender, most 

 flexible, and relatively smaller and less strong than in the other 

 genera. 



Pelvic Girdle and Extremity. 



The pelvic girdle presents more characteristic generic differ- 

 ences than do the bones of the pectoral girdle, though in all 

 the structure is of the same plan. The rod-like ilium ends in a 



