BRITISH FOSSIL REPTILES. 85 



the formation of the foramen ovale, is relatively shorter than in 

 the ischia of any of the preceding species ; and the neck of the 

 bone, or that end or angle which enters into the formation of 

 the acetabular cavity, is relatively shorter and broader. The 

 corresponding part of the scapulo-clavicular bone, vrhich cor- 

 responds with the ischium, presents the same distinctive cha- 

 racter. The acetabular surface is rough, convex, and with a 

 ridge along the middle of its long axis. 



Plesiosaurus trochanterius. 



The long bones of the second gigantic Plesiosauroid species, 

 from the Kimmeridge clay, deviate from the usual structure of 

 the humerus and femur in that genus in having a strongly- 

 developed trochanterian ridge projecting from the outer side of 

 the head of the bone : this process is of considerable breadth, 

 stands well out from the surface at its upper part, then gradu- 

 ally subsides, and is lost in the upper third of the humerus. 

 The shaft of the bone is more cylindrical than in most of the 

 Plesiosauri, and the distal expanded extremity is of less rela- 

 tive breadth. The whole surface of the bone is roughened by 

 longitudinal furrows, ridges and foramina; and on the inner 

 side of the bone, about one fourth of the length of the bone 

 from its head, there is a transversely elongated very rough sur- 

 face for the implantation of muscle. 



One of these long bones in the museum of Viscount Cole mea- 

 sures two feet in length, nineteen inches in circumference at the 

 head, including the trochanter, twelve inches and a half round 

 the middle of the shaft, and ten inches across the flattened 

 distal end ; this is terminated by two slightly concave equal 

 articular surfaces, which meet at a ridge or salient angle. The 

 thickness of these surfaces is one inch nine lines ; they are 

 each traversed longitudinally by a convex ridge, the base of 

 which is rather more than two thirds the breadth of the arti- 

 cular surface, which is slightly concave on each side of the 

 median ridge. 



Locality. — This bone is from the Kimmeridge clay of Shot- 

 over Hill. The trochanter here rises as high as the head 

 itself, from which it is separated by a deep and narrow 

 groove. 



Professor Sedgwick possesses a humerus or femur of this 

 species, from the brown alluvial clay at Bourn, Cambridge, pre- 

 senting a single long external trochanter, from which the bone 

 suddenly tapers to the shaft, and then becomes flattened and 

 expanded. 



