FOSSIL REPTILES OF DORSET. 17 



anterior part of a jaw about eleven inches in length ; both the right 

 and left rami of which shew twelve alveoli with slightly 

 compressed teeth, bearing only a few longitudinal ridges; a con- 

 siderable portion of the posterior end of the jaw is wanting. The 

 second, third, fourth, and fifth alveoli are much larger than the 

 others ; to accommodate which the jaw is wider than where the 

 alveoli are smaller. The bases of the largest teeth had a diameter 

 of at least half-an-inch, and must have projected about an inch and 

 a quarter beyond the margin of the jaw. 



ORDER ICHTHYOPTERYGIA, Owen. 



This Order includes only the fish-like Ichthyosaurus. Both 

 this and Pleiosaurus were air-breathers, and their nostrils similarly 

 placed at the top of the head. The vertebrae were amphicoelous 

 (hollow at both ends), the phalangal-bones encased in a cover- 

 ing similar to the fin of a whale ; the jaws and teeth shew they were 

 carnivorous ; the remains of fish and reptiles which have been 

 found in their skeletons testify to the nature of their food. The 

 caudal vertebrae are compressed towards the end of the tail and 

 vertically flattened, so as to give it a lateral movement from 

 side to side. 



GENUS ICHTHYOSAURUS, Konig. 



The Ichthyosaurus resembled the Cetacea in the apparent absence 

 of a neck, through the similarity of the breadth of the base of the 

 head with the part of the body to which it was attached. It had 

 doubtless a horizontal caudal fin to facilitate its coming to the 

 surface more rapidly, which in an air-breather was necessary, but 

 being cold-blooded there was no need of its doing so as 

 frequently as the warm-blooded whale. The nostrils of the 

 Ichthyosaurus are placed a little in front of the orbits ; it is 

 destitute of sacrum, sternum, and sternal ribs ; the teeth are not 

 placed in sockets, but in a common alveolar groove ; the swimming 

 paddles are composed of a number of polygonal phalangal-bones 

 arranged in five longitudinal rows with a marginal row of ossicles 

 at each border. The bodies of the vertebrae are usually detached 



