xiii. PLEIOSA URUS. 351 



A broad, thin, piano-undulate bone, with an outline formed by 

 two convex and two concave curves, the longest curve being a 

 convex symphysial sweep, separated by concave outlines from a long 

 narrow convex articulating face. The bone shews a central point 

 of radiating vascular structure. The articulating face is above 6 

 inches long, and 3*0 deep : the deepest part of the symphysis 

 i -4 deep ; the bone generally from 0-6 to 0-3. From the middle of 

 the articulating face to the opposite margin, 12*0 inches; from 

 the anterior part of the symphysis to the posterior and inner angle, 

 13-5 (Diagram CXLV.). 



Diagram CXLV. Ischium of Pleiosaurus. One-tenth of nature. 



It seems probable that this bone may be an ischium. It more 

 resembles the ulnar or fibular bone of an ordinary plesiosaurus, but 

 that would require the admission of a paddle 2/0 or more inches 

 broad, which could not be adjusted to any femur or any phalangals 

 at present known. 



A larger outline of this bone may, conjecturally, be placed in 

 relation to the pubis already described, as in Diagram CXLIV. 

 A bone quite similar, and of the same dimensions, is described 

 by Professor Owen in his Report on Fossil Reptiles for 1839. 



Bones of the extremities. There are portions of four first-bones 

 of the leg, two of them nearly complete, among the instructive 

 specimens from Market-Rasen. These are much alike, but one is 

 larger than the other. The longer and larger bone may be assumed 

 to be a femur, the smaller to be a humerus. The proximal end is 

 injured in each case, but there is a separate fragment of a spheroidal 

 shape and measuring 7-5 by 7-4 inches. There is no projecting 

 trochanter. From this bulky head the shank proceeds in a 

 sub-cylindrical form, and then passes, by continual thinning and 



