Williston.] Mosasaurs. 157 



cavity. The outer distal angle of the bone is broadly rounded. 

 The second, third and fourth metacarpals have the proximal 

 extremity broadly convex transversely and much expanded ; 

 the shaft is constricted beyond the middle into a flattened cyl- 

 inder. The distal extremity is moderately expanded and 

 squarely truncate. The third is a little shorter than the second, 

 the fourth distinctly shorter than the third. The fourth may 

 be recognized by the less symmetrical proximal extremity and a 

 shallow emargination of the inner border distally. The fifth 

 metacarpal is short and flattened. The proximal extremity is 

 nearly transverse to the long axis, and but little convex. Its 

 position and mode of articulation with the carpus indicate less 

 lateral movement than exists in the other metacarpals. The 

 distal extremity is much broader, the longer border for the 

 articulation of the phalanx placed obliquely to the axis of the 

 bone. The inner border is straight or gently concave ; the 

 outer border is shorter and deeply concave. 



Platecarpus. The metacarpals of Platecarpus are less differen- 

 tiated than in Clidastes or Mosasaurus. The first is somewhat 

 flattened and is more expanded proximally than the others, its 

 proximal articular surface being placed somewhat obliquely to 

 the long axis of the bone. The fourth has a slight sigmoid 

 curvature, and the fifth is shaped somewhat like the corre- 

 sponding bone of Clidastes, with the broader extremity distal. 

 The second, third and fourth are more expanded proximally 

 than distally, but the discrepancy is not as great as in Clidastes. 

 The proximal extremities are not as convex transversely, and 

 all the bones are separated more widely by cartilage. 



Tylosaurus. The metacarpals are more slender in Tylosaurus 

 than in the other genera. The first is the broadest and stoutest, 

 and as long as the second. It is more expanded proximally 

 than distally, but not much. The ends are nearly transverse 

 to the long axis, and but slightly convex. The inner border is 

 more deeply concave than the outer. The second, third and 

 fourth metacarpals are slender bones, scarcely distinguishable 

 in shape from the proximal phalanges. They are only moder- 



