196 EEPTILIA. 



Tortoises, is elongated and more or less cylindrical, and is either 

 altogether apodal, as in the Snakes, or is provided with two or four 

 extremities, which as a rule serve only to support and push on the 

 body which glides along the ground on its belly. In correspondence 

 with this mode of locomotion a cervical region is scarcely at all 

 marked, and even when more developed is always relatively rigid ; 

 the tail on the other hand is long and movable. 



The skin, as opposed to the predominating soft and naked skin of 

 the Amphibia, is tough and firm, in consequence of ossifications of 

 the cutis as well as of a cornification of the epidermis. The former 

 may give rise to bony scutes, overlapping one another in a tectiform 

 manner (Scincoidea), or to larger bony plates, which constitute a 

 hard, more or less continuous, dermal armour (Crocodiles, Tortoises). 



In general, pigments are present in the dermis as well as in the 

 deeper layer of the epidermis ; they determine the diverse colouring 

 of the skin, and sometimes cause a true change of colour (green 

 Tree Snake, Chamceleon). Cutaneous glands are also widely distri- 

 buted among the Reptilia. Many Lizards in particular possess rows 

 of glands on the inside of the femur and in the anal region, which 

 open by distinct pores sometimes on wart-like protuberances (femoral 

 pores, anal pores). In the Crocodiles, too, larger groups of glands 

 are placed beneath the dermal armour both at the sides of the anus 

 and on the sides of the rami of the lower jaw. 



The skeleton only exceptionally presents the embryonic form of a 

 cartilaginous cranial base and persistent notochord. The vertebral 

 column is more distinctly divided into regions than is that of the 

 Amphibia, although the thoracic and lumbar regions still allow of no 

 sharp limitation. In the cervical region the first vertebra becomes 

 the atlas and the second the axis. While fossil Hydrosaurians and 

 the Ascalabota possess biconcave vertebrae, the vertebral bodies of 

 other Reptiles are always bony and generally proccelous. 



Ribs are very generally present, often along the whole length 

 of the trunk. In the Snakes and the snake-like Lizards, 

 in which a sternum is absent, all the vertebrae of the trunk 

 with the exception of the atlas bear ribs, which, to compensate 

 for the absence of limbs, are capable of free movements. In the 

 Lizards and Crocodiles (fig. 573) there are short cervical ribs. The 

 thoracic ribs are joined to a sternum by means of special sternocostal 

 pieces. In the Crocodiles there is in addition an abdominal sternum, 

 which extends over the belly to the pelvic region, and is composed 

 of a number of ventral ribs (without dorsal part). The sacral 



