ICHTHYOPTERYGIA 203 



could not be brought with the required rapidity to the surface 

 to respire ; but the Ichthyosaurs, not being warm-blooded or 

 quick breathers, would not need to bring their head to the 

 surface so frequently or so rapidly as the whale ; and more- 

 over, a compensation for the want of horizontality of their tail 

 fin was provided by the addition of a pair of hind paddles, 

 which are not present in the whale tribe. The vertical fin 

 was a more efficient organ in the rapid cleaving of the liquid 

 element, when the Ichthyosaurs were in pursuit of their prey, 

 or escaping from an enemy. 



The general form of the cranium of the Ichthyosaurus 

 resembles that of the ordinary cetaceous dolphin (Delphinus 

 tursio) ; but the /. tcnuirostris rivals the Delphinus gangeticus in 

 the length and slenderness 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 zygomatic 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 single though 

 compound 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 of the nostrils, 

 in the immense size of the orbits, and in the large and nume- 

 rous 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 



* The anatomical reader is referred to the w liter's " Report on British Fossil 

 Reptiles," Trans Brit. Assn.-. 1839, and to the Annals and Magazine of Natural 

 History, 1858 p. 388. 



