xii PHYLUM CHORDATA 437 



In some (Land and Fresh-water tortoises) they are provided 

 each with five free digits terminating in curved horny claws ; 

 in the Turtles the digits are closely united together, and the 

 limb assumes the character of a " nipper " or swimming 

 paddle. The cloacal aperture is longitudinal. 



The Crocodilia, the largest of living Reptiles, have the 

 trunk elongated and somewhat depressed, so that its breadth 

 is much greater than its height. The snout is prolonged, 

 the neck short, the tail longer than the body and compressed 

 laterally. The limbs are relatively short and powerful, with 

 five digits in the manus and four in the pes, those of the 

 latter being partly or completely united by webs of skin. 

 The eyes are very small ; the nostrils are placed close to the 

 end of the snout and capable of being closed by a sphincter 

 muscle. The cloacal aperture is a longitudinal slit. 



Characteristic of the Squamata is the development in the 

 epidermis of horny plates, the scales, which cover the entire 

 surface, overlapping one another in an imbricating manner. 

 Sometimes they are similar in character over all parts of the 

 surface : usually there are specially developed scales the 

 head shields covering the upper surface of the head. In 

 the majority of Snakes the ventral surface is covered with a 

 row of large transversely elongated scales, the ventral shields. 

 In some Lizards (Chameleons and Geckos) the scales are 

 reduced and modified into the form of minute tubercles or 

 granules. In some Lizards special developments of the 

 scales occur in the form of large tubercles or spines. 



In the Snake-like Amphisbaenians there are no true scales 

 with the exception of the head shields, but the surface is 

 marked out into annular bands of squarish areas. 



In addition to the modification of the scales, the integu- 

 ment of the Chameleons is remarkable for the changes of 

 colour which it undergoes, these changes being due to the 



