144 REPTILIA . class hi 



The appendicular skeleton is constituted essentially as in Amphibians. 

 In the fore-limb the humerus of some fossil forms is pierced at its distal end 

 bv one or two foramina, as in certain mammals ; but among living reptiles 

 this character appears only in Sphenodon. The ulna sometimes exceeds the 

 radiusin length, and is sometimes provided proximally with an olecranon process. 

 The carpus consists always of two rows of bones, of which the proximal contains 

 two or three, and the distal from three to six elements ; occasionally these 

 rows are separated by one or two centralia. The bones of the manus undergo 

 excessive modifications correlating w4th their function. There are commonly 

 from two to five digits, and among Ichthyosaurs even more than five. The 

 third and fourth digits usually possess a larger number of phalanges than the 

 rest. The most generalised type of manus and pes is found among 

 Chelonians. 



Numerous modifications occur also in the pelvic region and posterior limb. 

 A sacrum, consisting normally of two vertebrae, is present in all reptiles with 

 the exception of Snakes, Ichthyosaurs, and Pythonomorphs. Among Snakes 

 the pelvis is either entirely wanting, or represented by vestigial ischia ; and 

 rudimentary ilia are present among apodal Lizards. All other reptiles have a 

 pelvis composed of the three usual elements, and as a rule they all take part 

 in the formation of the acetabulum. The ilium is often much extended 

 antero-posteriorly. The ischia are generally larger than the pubes, and both 

 pairs converge ventrally in front, and join in median symphyses. A decided 

 bird-like aspect is observed in the pelvis of Dinosaurs, owing to the downward 

 and backward extension of the greatly elongated ischium, and the anterior 

 prolongation of the dorsal part of the ilium. The hind limb closely resembles 

 the fore in most cases. One or two trochanters may occur on the proximal 

 portion of the femur, and there are generally two rows of tarsals. The 

 Sauropsida (Reptiles and Birds) differ from mammals in having the ankle-joint 

 between the two rows of tarsals, and never between the tibia and proximal 

 tarsals. Various modifications of the hind limb occur, which are noted in the 

 discussion of the different groups. 



Habitat. — Their respiration being exclusively pulmonary, reptiles are 

 adapted to a terrestrial or amphibian existence. Even the small number of 

 marine Chelonians and Lacertilians visit the shores periodically for the pur- 

 pose of depositing their eggs. Among extinct forms, Ichthyosaurs, Sauro- 

 pterj'-gians, and Pythonomorphs were truly marine, and provided with j)addle- 

 shaped extremities. The majority of fossil reptiles, however, were land forms ; 

 some appear to have been arboreal, and the Pterosaurs, as their name implies, 

 were volant, or at least capable of gliding through the air. There is reason to 

 suppose that Dinosaurs were viviparous, as is known to be true of Ichthyo- 

 saurs, and some of the former Avere of bipedal gait. 



Over 3500 recent species of reptiles have been described, a number greatly 

 exceeding the known fossil forms. The latter, however, exhibit far greater 

 diversit}^ in structure than living species, and hence a knowledge of their 

 organisation is indispensable for an understanding of the class. The earliest 

 undoubted reptilian remains (Proterosauria) are of Lower Permian age, and 

 although not plentiful here, numerous forms are known from the Upper Per- 

 mian of Europe, North America, and South Africa. The culmination of the 

 class occurred during the Jura-Trias, after which Eeptiles steadily declined. 



Taxonomij. — The separation between Reptiles and Amphibians was first 



