592 



RE PT ILIA. 



Some of the main contrasts between living Reptiles and 

 Birds are summarised in the following table :- 



REPTILES. 



BIRDS. 



The exoskeleton consists of horny 

 epidermal scales, sometimes augmented 

 by bony dermal scutes. 



The centra of the vertebra are rarely 

 like those of birds. 



When there is a sacrum, its vertebrae 

 (usually two in number) have large ex- 

 panded ribs with the ends of which the 

 ilia articulate. 



The cartilaginous sternum may be- 

 come bony, but is not replaced by 

 membrane bones, unless perhaps in 

 Pterodactyls. 



When there is an interclavicle or epi- 

 sternum, it remains distinct from the 

 clavicle and sternum. 



The hand has more than three digits, 

 and at least the three radial digits are 

 clawed. 



In living reptiles the ilia are prolonged 

 further behind than in front of the aceta- 

 bulum ; the pubes slope downward and 

 forward ; there are usually pubic and 

 ischiac symphyses. 



There are often five toes ; the tarsals 

 and the metatarsals remain distinct. 



At least two aortic arches persist ; 

 only the Crocodilia have a structurally 

 four-chambered heart ; more or less 

 mixed blood always goes to the pos- 

 terior body. 



The body has approximately the tem- 

 perature of the surrounding medium. 



The optic lobes lie on the upper 

 surface of the brain. 



There is an outer covering of feathers, 

 and though there may be a few scales, 

 there are never scutes. 



The centra of the cervical vertebrae 

 have usually a saddle-shaped terminal 

 curvature. 



The two sacral vertebrae have no 

 expanded ribs, they fuse with others 

 to form a long composite " sacrum," 



The cartilaginous sternum is replaced 

 by membrane bone from several centres. 



When there is an interclavicle, it is 

 confluent with the clavicles. 



The hand has not more than three 

 digits, and at most two digits are 

 clawed. The fore-limbs are modified 

 as wings ; some carpals fuse with the 

 fused metacarpals. 



The ilia are greatly prolonged in front 

 of the acetabulum, the inner wall of 

 which is membranous. The pubes slope 

 backwards, parallel with the ischia ; 

 only in Struthio is there a pubic 

 symphysis, only in Rhea. is there an 

 ischiac one. 



There are not more than four toes ; 

 the proximal tarsals unite with the 

 tibia, forming a tibio-tarsus ; the first 

 metatarsal if present is free, but the 

 three others are fused to one another 

 and to the distal tarsals, forming a 

 tarso-metatarsus. 



There is but one aortic arch, to the 

 right ; the heart is four-chambered ; 

 the blood sent to the body is purely 

 arterial. 



The body temperature is high and 

 almost constant. 



The optic lobes lie on the sides of the 

 brain. 



The lungs have associated air-sacs. 



The sutures between the bones of the 

 skull are usually obliterated at an early 

 stage. 



The right ovary atrophies. 



