ICHTHYOSAURIA. 



391 



Sub-elass 6. Ichthyosauria. 



Very peculiar large extinct fish-like marine reptiles with biconcave verte- 

 brae, four paddle-shaped limbs with very short long-bones ; a large nuiriber 

 of phalanges and often more digits than five, with pineal foramen, pterygoids 

 reaching forward to the vomers, and fixed quadrate. Trias to Cretaceous. 



The Ichthyosauria were large fish-like reptiles, which inhabited the 

 seas of a considerable part of the earth during the secondary period. 

 Their remains are found in Europe, Africa, America, AustraUa, New 

 Zealand, and India. Some of the larger kinds appear to have reached 

 a length of from 30 to 40 feet. They are distinguished externally by the 

 large size of the head and of the eyes (Fig. 217), by the absence of a neck, 

 by the two pairs of flipper-like appendages, by the dorsal median fins, 

 and by the vertical caudal fin, into the lower lobe of which the vertebral 



Fia. 217. — Ichthyosaurus guadriscissus. A Fossilised skeleton. B Outline restoration show- 

 ing outline of integument with dorsal and caudal fins. L. Jurassic (U. lias), Wiirtemburg 

 (after E. Fraas, from Woodward). 



cohman is prolonged. The size and form of the head, the absence of the 

 neck and the form of the anterior Umb give them a certain superficial 

 resemblance to whales. They have indeed been spoken of as the whales 

 of the Mesozoic seas. That they are true reptiles must be considered 

 certain ; but they stand far apart from other reptiles and cannot be said 

 to show resemblances to one sub-class more than to another. By the 

 presence of a pineal foramen in the roof of the skull and the form of the 

 pectoral girdle they resemble the Uzard group ; by the anterior extension 

 of the pterygoid to the vomers, the Rhynchocephalia ; and by the form 

 of the temporal arcade they recall the Anomodontia and Chelonia. 



In the zoological language of the day tliis isolation in structure with regard 

 to other reptiles is expressed by sajdng that their descent is unknown ; 

 but in this they do not differ from other reptilian groups, of none of which 



