Williston.] Mosasaurs. 165 



transverse in the middle, rounded on either side. The inner 

 part has a short, thickened facet for articulation with the large 

 tarsal bone. The middle portion is somewhat thinner and has 

 a facet for union with one of the smaller tarsals. The outer 

 part of the border gives attachment to cartilage and meets the 

 lateral border in an obtuse angle. The proximal end is oval in 

 cross-section, the articulation placed obliquely to the long axis, 

 with the inner angle rounded. 



Platecarpus. The fibula of Platecarpus is not unlike the cor- 

 responding bone of Clidastes, but is less expanded distally and 

 more symmetrical. The outer border is moderately and nearly 

 evenly concave throughout. The inner border is more deeply 

 concave, with its end slightly convex. The proximal end of the 

 bone is about two-thirds the width of the distal and is nearly 

 transverse. The distal end is somewhat oblique, the inner 

 angle expanded. There is a small facet near the outer end of 

 the border for the tarsal. 



Tylosaurus prorigt r. The fibula in this genus is very small in 

 proportion to the tibia, its extremities but little expanded. The 

 lower extremity is expanded more than the upper ; the sides are 

 concave, with the greater concavity on the inner side. 



Tarsus. 



Clidastes. Of Clidastes velox there is but a single tarsal bone 

 preserved. Of C. westii there are three or four. The largest is 

 the one articulating with tibia and fibula. The broader tibial 

 extremity has in the proximal side a large oval facet for the tibia, 

 immediately internal to which the bone is directed proximad, 

 with a free oval border to the distal external angle of the fibula. 

 Nearly at right angles to this free border there is an oval articu- 

 lating facet for the fibula. The outer and distal border is 

 rounded and apparently articulates with no bones. On the in- 

 ner side there are two articular facets, indistinctly separated for 

 the smaller tarsal bones. These two bones, in C. westii, resem- 

 ble very much the distal carpals. They are irregularly four- or 

 five-sided, the proximal one articulating with the fibula, the 

 outer and distal tarsals, and in part with the fifth metacarpal. 



