108 INTRODUCTION TO CLASSIFICATION. 



The dorsal vertebrae are immovably connected, and have no 

 transverse processes, the proximal ends of the ribs uniting 

 directly with the vertebrae. The spinous processes of most of 

 the dorsal vertebrae expand into the median or " neural " plates 

 of the carapace, while the ribs of these vertebrae enlarge into 

 its lateral or " costal " plates. All these plates become united by 

 sutures, so that the ribs are immovable. There are no sternal 

 ribs, nor any sternum ; the place of the latter being in part 

 taken by the plastron, which usually consists of nine pieces, 

 four pairs and one antero-median. 



There are two vertebrae in the sacrum. 



All the bones of the skull, except the mandible and the 

 hyoid, are immovably united together. 



The presphenoid cartilage forms an interorbital septum. 



Both pair of limbs are well developed. 



The jaws are ensheathed in a horny beak, and there are no 

 teeth. 



The heart is three-chambered, with a partial division of 

 the ventricle, as in the two preceding orders. There is a large 

 urinary bladder, and the males have a single grooved penis 

 attached to the front wall of the cloaca. 



These are all the orders of existing Reptilia, the following 

 orders existing only in the fossil state 



1. The ICHTHYOSAURIA. No exoskeleton is known. The 

 centra of the vertebras are short, broad and bi-concave, and the 

 arches remain distinct from them throughout life. Transverse 

 processes are represented only by small elevations of the centra. 

 Those in the dorsal region are double, and articulate with the 

 proximal ends of the ribs, which are deeply divided into distinct 

 capitula and tubercula. 



There is no sacrum. No sternal ribs, nor sternum, are 

 known, but false abdominal ribs are developed as in the Cro- 

 codiles. 



The skull has huge orbits, separated by an interorbital 

 septum ; and a long and tapering snout, formed chiefly by the 

 premaxillae. The nostrils are placed close to the orbits. There 

 is a bony ring in the sclerotic. 



