REPTILIA. ICHTHYOSAURIA. 195 



Suborder (3). PLEURODIRA. 



The carapace and plastron are strongly ossified, and firmly 

 united to the pelvis. The head and neck can be folded late- 

 rally under the carapace, but cannot be drawn back under it. 

 The cranium has a more or less complete false roof, and in the 

 mandibular articulation the knob is borne by the cranium, and 

 the cup by the mandible. Chelys is a well-known genus. 



Order 4. ICHTHYOSAURIA 1 . 



The order includes a number of large extinct marine 

 reptiles whose general shape is similar to that of the Cetacea. 

 The skull is enormously large, and the neck short. The tail 

 is very long, and is terminated by a large vertically-placed 

 bilobed fin, the vertebral column running along the lower 

 lobe. The very numerous vertebrae are short and deeply bi- 

 concave. The vertebral column can be divided into caudal 

 and precaudal regions only, as the ribs which begin at the 

 anterior part of the neck are continued to the posterior end 

 of the trunk without being connected with any. sternum or 

 sacrum. The precaudal vertebrae bear two surfaces for the 

 articulation of the ribs, while in the caudal vertebrae the 

 two surfaces have coalesced. The caudal region is also 

 distinguished by its chevron bones. The vertebrae have 

 no transverse processes, and the neural arches are not 

 firmly united to the centra, and have only traces of zyga- 

 pophyses. The atlas and axis are similar to the other 

 vertebrae, but there is a wedge-shaped intercentrum be- 

 tween the atlas and the skull, and another between the 

 atlas and the axis. The skull is greatly elongated (fig. 32) 

 and pointed, mainly owing to the length of the premaxillae. 

 The orbits are enormous, and there is a ring of bones in the 

 sclerotic (fig. 32, 15). The anterior nares are very small ; and 



1 R. Lydekker, Nat. Sci. vol. i. p. 514, 1892. Further references are 

 there given. 



132 



