ROOM III. PLESIOSAURUS DOLICHODEIRUS. 345 



inferior aspect of the centrum ; and they are rather wider 

 than long. The annular part is united to the body by suture, 

 but not anchylosed to it. The spinous process is rather 

 elevated ; the posterior zygapophyses are higher than the 

 anterior, and rest almost horizontally on those of the conti- 

 guous vertebrae. 



The anterior cervical vertebrae support small ribs, which are 

 articulated by two tubercles, and terminate in a hatchet-form, 

 like the analogous elements in the Crocodile. In the six 

 succeeding vertebrae, the ribs are elongated, and assume by 

 degrees the form of the dorsal costal-processes. 



The vertebras of the tail are distinguished by the small 

 facets for the haemapophyses, which, as in the crocodiles, are 

 articulated between the junction of two vertebrae, so that 

 there are two articular depressions for each of the laminae, the 

 centrum having four, viz. two at the anterior edge and two 

 at the posterior ; the transverse processes of the caudals are 

 attached by suture as in the young crocodile. 



The humero-pectoral arch in the Plesiosaurus is very 

 remarkable for the great size, and antero-posterior expansion 

 of the coracoids, (see Lign. 73). 



The structure of the ribs is also peculiar ; for each pair of 

 costal processes formed an osseous cincture, which encircled the 

 body, and was composed of five distinct parts ; namely, the 

 two spinal, which were articulated to the centrum by a bifur- 

 cated head ; and three slender vertical bones that were fitted 

 to one another by oblique grooves ; the median piece being 

 transversely elongated, and slightly curved and pointed at 

 both extremities ; these intermediate processes are termed by 

 Mr. Conybeare the " sterna-costal arcs;" a similar structure 

 exists in the Chameleons and certain species of Iguanas. This 

 mechanism admits of great expansion of the abdominal 

 muscles and integuments during the inflation of the lungs ; 

 and hence M. Cuvier suggested the probability that the 

 pulmonary organs in the Plesiosaurus were very large, and 

 that this marine saurian, like the Chameleon, changed the 

 colour of its skin according to the varied intensity of its 

 respiration. 



The principal bones of the anterior extremities, or paddles, 

 consist of the usual normal elements, viz. a humerus, radius, 

 and ulna ; the first is a stout bone, with a rounded head, and 



