ICHTHYOPTERYGIA. 223 



resembles that of the ordinary cetaceous dolphin (LeI/phinus 

 tursio) ; but the /. tenuirostris rivals the Delphinus gangeticus 

 in the length and slenclerness of the jaws. The essential 

 difference in the sea-reptile lies in the restricted size of the 

 cerebral cavity, and the vast depth and breadth of the zygo- 

 matic arches, to which the seeming expanse of the cranium is 

 due ; still more in the persistent individuality of the elements 

 of those cranial bones which have been blended into such 

 single bones in the sea-mammal. The Ichthyosaurus further 

 differs in the great size of the premaxillary and small size of the 

 maxillary bones, in the lateral aspects and antorbital position 

 of the nostrils, in the immense size of the orbits, and in the 

 large and numerous sclerotic plates, which latter structures give 

 to the skull of the Ichthyosaurus its most striking features. 



The true affinities of the Ichthyosaur are, however, to be 

 elucidated by a deeper and more detailed comparison of the 

 structure of the skull ; and few collections now afford richer 

 materials for pursuing and illustrating such comparisons than 

 the palseontological series in the British Museum* The two 

 supplemental bones of the skull, which have no homologues 

 in existing Crocodilians, are the post-orbital and super-squa- 

 mosal ; both, however, are developed in Archcgosaurus and 

 the Labyrinthodonts. The post-orbital is the homologue of 

 the inferior division of the post-frontal in those Lacertians— 

 e.g., Iguana, Tegus, Ophisaurus, Anguis, in which that bone is 

 said to be divided ; but in Ichthyosaurus it more resembles a 

 dismemberment of the malar. Its thin obtuse scale-like lower 

 end overlaps and joins by a squamous suture the hind end of 

 the malar : the post-orbital expands as it ascends to the middle 

 of the back of the orbit, then gradually contracts to a point as 

 it curves upward and forward, articulating with the super-squa- 



* The anatomical reader is referred to the writer's "Eeport on British 

 Fossil Reptiles," Trans. Brit. Assoc. 1839, and to the Annals and Magazine of 

 Natural History, 1858, p. 388. 



