292 ZOOLOGY SECT. 



on the basi-occipital. The basi-sphenoid is a well developed bone. 

 The mandible articulates with the skull through the intermediation 

 of a quadrate, and consists of five or six bones on each side. The 

 ankle-joint is an articulation between the proximal and distal 

 divisions of the tarsus. As in the Amphibia there is a cloaca into 

 which the rectum and the renal and reproductive ducts open. The 

 heart consists of two auricles and a ventricle which is sometimes 

 incompletely, sometimes completely, divided into two parts. 

 Branchiae are never present at any stage. The mesonephri are 

 never the functional renal organs of the adult, but are always 

 replaced by metanephri. Both an amnion and an allantois are 

 present in the embryo, the latter becoming highly vascular and 

 acting as a temporary foetal organ of respiration. 



The class Reptilia comprises four orders having living repre- 

 sentatives, in addition to a number of extinct groups. In the 

 Mesozoic period the class reached its maximum both in the number 

 of its representatives and the size which many of them attained ; 

 at that period they were very unmistakably the dominant class 

 of the Animal Kingdom. In the Tertiary period they underwent 

 a decline, while the Birds, and, in a yet higher degree, the 

 Mammals, were gaining a preponderance over them. The living 

 Reptiles are the Lizards and Chamseleons, the Tuataras, the 

 Snakes, Tortoises and Turtles, and the Crocodiles and Alligators. 

 Though horny epidermal scales are not by any means present in 

 all the Reptiles, their occurrence as a complete covering is 

 characteristic of the group and peculiar to it. When scales are 

 not present, the epidermis is always hardened and cornified so as 

 to form plates of horny material, such as the horny plates of the 

 Tortoises, which protect the underlying soft parts from injury and 

 desiccation. Bony plates are frequently present as well. In most 

 respects the internal structure of the Reptilia shows a very decided 

 advance on that of the Amphibia. The skull is more completely 

 ossified, as well as the pectoial and pelvic arches, and both vascular 

 and nervous systems show a higher grade of organisation. 



1. EXAMPLE OF THE CLASS. A LIZARD (Lacerta). 



The most striking external differences between the Lizard (Fig. 

 905) and the Frog are the covering of scales, the comparative 

 smallness of the head, and the presence of a distinct neck, the great 

 length of the caudal region, the shortness of the limbs, and the 

 approximate equality in length of the anterior and posterior pairs. 

 The anterior limbs are situated just behind the neck, springing 

 from the trunk towards the ventral surface. The fore-limb, like 

 that of the Frog, is divided into three parts, the upper-arm or 

 brachium, the fore-arm or awti-brachium, and the hand or mcmus ; 

 there are five digits provided with horny claws, the first digit or 



