86 REPORT— 1839. 



Foot. Inches. 



Length ......... 1 4 



Breadth of distal extremity ... 9 

 middle of shaft ... 4 



The proximal surface is pitted like the epiphysial end of a 

 mammiferous femur : it was probably capped by cartilage, and 

 joined by ligamentous substance, without a synovial joint, to 

 the acetabulum. 



Plesiosaiirus affinis. 



In the excellent collection of fossil remains in the museum of 

 Viscount Cole there is a humerus or femur similar to that of 

 the preceding species in regard to the existence of a trochanter, 

 but differing in its smaller development, in the general form of 

 the shaft of the bone, and in size : this bone is only eight 

 inches in length. 



The trochanter projects from the outer side of the head of the 

 bone, but its most prominent part is on a level with the inferior 

 margin of the head or proximal articular surface. The breadth 

 of the trochanter is rather more than one third the breadth of 

 the proximal extremity. The trochanter gradually subsides to 

 the level of the shaft, which in the upper fourth of its extent 

 presents with the trochanter a triedral transverse section with 

 the angles rounded off. The shaft begins to be flattened imme- 

 diately below the trochanter, and gradually to increase in breadth, 

 but it preserves a greater relative thickness than in the larger 

 bone. The general surface is broken by fine striae and perfora- 

 tions, and there is a well-marked transversely oblong rugosity 

 on the inner side of the upper fourth of the bone. 



The differences just specified between this small trochanterian 

 bone and the great one before described show that it cannot have 

 belonged to a younger specimen of the same species. Both bones 

 are solid throughout. 



Locality. — Kimmeridge clay, Heddington-pits, Oxford. 



CHARACTERS OF THE GENUS ICHTHYOSAURUS. 



The Enaliosaurians of the present family differ from those of 

 the preceding most remarkably in the shortness of the neck and 

 the equality of the width of the occiput with that of the thorax, 

 which almost immediately succeeds it ; impressing the observer 

 with the conviction that the recent animal must have resembled 

 a Cetacean or a Fish in the total absence of any cervical con- 

 striction. 



This close approximation in the Ichthyosauri to the form of 

 the most strictly aquatic animals of the existing creation is ac- 



