342 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



the extraordinary length of the neck, and the relative small- 

 ness of the head. The neck, which in most animals is formed 

 of but five vertebrae, and in the extremest recent example, the 

 Swan, does not exceed twenty-four, is in certain species of 

 Plesiosaurus composed of from twenty to forty vertebrae, and 

 is four times as long as the head, and equal in longitudinal 

 extent to the body and tail ; while the length of the head (in 

 P. dolichodeirus) is less than one-thirteenth of the entire 

 skeleton ; the tail is very short. 



The skull resembles that of the Crocodiles in its general 

 form, but is proportionately smaller ; the breathing apertures 

 are situated anterior to the orbits, on the highest part of the 

 head. The orbit is relatively large, and furnished with a zone 

 of bony plates, as in the sclerotica of certain lizards and birds. 1 

 The lower jaw has the usual structure of the saurians ; but 

 the dentary bone is greatly expanded anteriorly, and united 

 in front. 



The teeth are implanted in separate sockets, and there 

 are from thirty to forty on each side the jaws. They are 

 conical, slender, long, pointed, slightly recurved, and lon- 

 gitudinally grooved from the base upwards, and have a long 

 round fang. The pulp-cavity is long and simple, surrounded 

 by a body of firm dentine, covered on the crown with a layer 

 of enamel, and at the base with cement. The dentition in 

 the Plesiosauri differs from that of the Crocodiles, in the 

 successional teeth emerging through distinct apertures on 

 the inner side of the sockets of their predecessors, and not J 

 through the pulp-cavity. 2 The vertebrae are relatively longer 

 than in the Ichthyosaurus, and their articular faces are either 

 flat, or slightly excavated towards the periphery, with a gentle 

 convexity in the centre. 



The Pectoral arch is remarkable for the greatly elongated 

 and broad Coracoid bones. The Ribs, which are very nume- 

 rous, and extend throughout a great portion of the vertebral 

 column, are connected, anteriorly, by slender bones; the 

 Ichthyosaurus has a similar structure. As these connecting 

 parts are so constructed as to admit of a certain degree of 

 gliding motion upon each other, it is inferred that consi-. 



1 See Dr. Buckland's " Bridge water Essay," PI. X. 



2 See Professor Owen's " Odontography," p. 282. 



