ORDER ICHTHYOPTERYGIA. 



1129 



of great width, and the third digit, or the one arising from the 

 intermedium, at least in the American genus, 

 consists of two longitudinal rows of bones. 

 The type species of Opthalmosaurus was of 

 moderate size, and occurs in the Oxford and 

 Kimeridge Clays ; it is characterised by the 

 inequality in the size of the three distal facets 

 of the humerus. In O. cantabrigiensis, of the 

 Cambridge Greensand, which may belong to 

 Baptanodon, these three facets have, however, 

 become nearly equal in size ; and this form 

 seems therefore to indicate the highest evolu- 

 tion of the order. It should be mentioned 

 that Professor Marsh and Mr Hulke differ from 

 Dr Baur in their interpretation of the three 

 bones of the second segment of the limb of 

 this genus ; correlating the middle bone with 

 the intermedium, and the postaxial one with 

 the ulna in the pectoral, and the fibula in the 

 pelvic limb. Finally, it should be observed 

 that a femur, from the Cambridge Greensand, 

 described under the name of Cetarthrosaurus, 

 and regarded as belonging to a member of the present order, but 

 which has been referred by Mr Hulke to the suborder Pythono- 



Fig. 1032. —Tooth of 

 Ichthyosaurus commun- 

 is; from the Lower Lias 

 of Gloucestershire. 



Fig. 1033.— Dorsal aspect of part of the right pectoral limb of Opthalmosaurus icenicus ', from 

 the Oxford Clay. One-third natural size, h, Humerus ; a, Trochanteric ridge of do. ; r, Radius ; 

 U, Ulna ; /, Pisiform. 



