108 REPORT— 1839. 



Ichthyosaurus communis, Conybeare. 



This species is characterized by its relatively large teeth, with 

 expanded or ventricose bases, and round, conical, slightly adun- 

 cate crowns, which taper more quickly and less regularly to the 

 apex than in the other species ; the apex is not very acute, and 

 the whole tooth is longitudinally furrowed, the base being sculp- 

 tured by coarse and deep grooves, with intervening convex 

 ridges. The upper jaw contains on each side from 40 to 50 

 teeth, of which 18 are implanted in the superior maxillary bone. 

 In the lower jaw there are on each side between 25 and 30 teeth. 

 The basal ridges in the large teeth are sometimes transversely 

 scored, and bifurcate as they approach the base, towards which 

 the bifurcations gradually diminish in size ; when the whole 

 may not unaptly be compared to the contracted head of a small 

 Pentacrinite. The anterior paddles are three times larger than 

 the posterior ones, and, as compared with the other known 

 species of Ichythyosauri, are relatively broader, and contain a 

 greater number of digital ossicles*. 



With respect to the size of this species, it appears to be se- 

 cond only to the Ich. jilatyodon. 



In the museum of Viscount Cole there is a head of the Ich- 

 thyosaurus communis which measures in length two feet nine 

 inches, indicating an animal of 20 feet in total length. The series 

 of teeth in the dentary bone of the lower jaw is one foot ten 

 inches and a half in extent. 



The head of this species is expanded posteriorly, but quickly 

 contracts to the base of the jaws, which are prolonged and 

 somewhat compressed ; towards their apices the profile ab- 

 ruptly converges to the tangential point. The basi-occipital 

 bone in this species differs from that of the Ich. platyodon in 

 having a shallower depression on the under part anterior to the 

 condyle, and its length is greater in proportion to its breadth. 



In the composition of the cranium may be noticed the very 

 small size of the median frontals, and the great share which 

 they have in the formation of the parietal foramen, of which 

 only the posterior angle lies in the interparietal suture. The an- 

 terior and posterior frontals form exclusively the upper bomid- 

 ary of the orbit. There are seventeen sclerotic plates in each 

 eye, the length of each plate being equal to half the diameter 

 of the central circular space. The orbit and eye are relatively 

 smaller than in the Ich. tenuirostris or platyodon. The nostril 



* Mr. Hawkins first appreciated this character of the present species ; he 

 states that " the metacarpus has eight bones ; the nine fingers contain no less 

 than two hundred and twelve." — Memoirs, &c., p. 28. He changed the name 

 assigned to the species by Mr. Conybeare, in order to express this pecu- 

 liarity. 



