xii. PLESIOSAURUS. 311 



analogies : nothing else is doubtful. One of the ischial bones is 

 so nearly complete as to fully justify the outlines of both, except 

 at the posterior edge. 



So few complete examples of these bones clearly separated from 

 the matrix have been carefully represented, that it seems worth 

 while to call attention to some of the points which mark the Oxford 

 plesiosaurus with a peculiar aspect. 



The pubis, preserving a large area in proportion to the ischium, 

 and meeting it so as to leave between the bones the doubly- 

 cycloidal interval nearly as usual, runs out into wings on the 

 anterior border, and from these deep curves return to the femoral 

 joint. The bones, generally thin, meet in a long, straight sym- 

 physis, with surfaces very deep in the posterior part, but shallow 

 toward the front. There is some intumescence in the posterior 

 part, not deserving the title of carination, on the outer or lower 

 side. 



The ischium is remarkable for the strong, sharp keel or angular 

 band, which runs from the acetabulum to near the posterior edge, 

 and for the lateral wings, which advance far forward, and are 

 separated from the acetabulum by a deep, sharp, inward curve. 

 The bone is vaulted below, and the symphysis is deepest anteriorly. 



Dimensions : P to 7, along the middle of the bones, 21 inches ; 

 pp. across the pubis, 21 ; ii. across the ischia, ai ; breadth between 

 the acetabula, 13. 



These dimensions are the average of two sets of bones belonging 

 to two animals. 



As yet no ilium has been discovered. 



The first bone of the hind-leg in the largest example is 11-9 

 inches long ; the extreme breadth at the distal end being 7 inches. 



The proximal extremity is 3-1 inches across in the general plane 

 of the bone, and at right angles to that 4*5, the difference being 

 caused by a large process answering to a trochanter. The bone 

 grows thinner, without much reduction of breadth for half its 

 length, and then widens and grows thinner to the distal extremity. 

 This extremity, formed in a general curve, has three facets, not 

 always clearly marked, for adaptation to the two bones which 

 represent tibia and fibula, the longest being anterior. 



The femur is succeeded by two broad angular bones (tibia and fibula) 

 in contact ; a circumstance observed in several of the paddles of 



