BOOM IV. STRUCTURE OF THE ICHTHYOSAURUS. 37 1 



posed and the bones disunited, but tept from dispersion, as 

 in a sack, till the whole mass was imbedded. 



STRUCTURE OF THE ICHTHYOSAURUS. The Ichthyosaurus, 

 though agreeing in its general characters with the Plesiosaurus, 

 presents a much nearer approach to the fishes than that 

 genus of Enaliosaurians. From the shortness of the neck, and 

 great breadth of the base of the cranium, its general outline 

 must have closely resembled that of a large Porpoise, or 

 Grampus, with enormous eyes, two pairs of fins or paddles, a 

 long tail, and, probably, a large integumentary caudal fin. 



The internal structure corresponds with the outer form in 

 its close approximation to fishes or cretaceans. The ver- 

 tebrae have their articular surfaces so deeply cupped, that it 

 is inferred " they were originally connected together by an 

 elastic capsule, filled with a fluid, as in the vertebral joints 

 of fishes, and the perennibranchiate, or most fish-like, of 

 Reptiles." 1 



The muzzle of the Ichthyosaurus is long and pointed ; the 

 lower jaw is formed of two branches, united anteriorly through 

 nearly half their length ; each branch is composed of six 

 bones, as in the Crocodiles and Lizards, but differently ar- 

 ranged than in those reptiles. 



The teeth are very numerous, amounting to nearly two 

 hundred in some species, and are placed in a single row on 

 each side the jaws, being implanted in a deep continuous 

 : groove without sockets. These teeth are of a pointed conical 

 form, longitudinally striated, with an expanded base. The 

 new teeth are developed at the inner side of the base of the 

 old ones, and grow up and displace them. The tooth con- 

 sists of a pulp-cavity, surrounded by a body of dentine, 

 which is invested at the base by a thick layer of cement, 

 and at the crown by a coat of enamel ; the pulp-cavity, in 

 fully-formed teeth, is more or less occupied by coarse bone. 2 

 The chief peculiarity of this structure consists in the inflec- 

 tion of the cement into vertical folds at the base of the 

 tooth, by which the marginal portion of the basal dentine is 

 divided into a corresponding number of processes, as in the 



1 Professor Ovren, on the Ichthyosaurus. " Brit Assoc. Rep." 1839, 

 p. 87. 

 Bee Prof. Owen's " Odontography," p. 275. 



