562 



REPTILES. 



by Professor Huxley, when he united the two classes as 

 Sauropsida, in contrast to Mammalia on the one hand, and 

 Ichthyopsida (Amphibians and Fishes) on the other. Let us 

 state some of the contrasts which he recognised, noting at the 

 same time that Reptiles form among Vertebrates a great 

 central assemblage, like " worms " among Invertebrates, 

 rather a number of classes than a class, exhibiting affinities 

 not only with Birds, but with Mammals and Amphibians as 

 well. 



Again we shall virtually quote from Huxley in noting some of the 

 distinctions between Reptiles and Birds : 



REPTILES. 



The exoskeleton consists of horny epi- 

 dermal scales, or of bony dermal scutes, 

 or of both. 



The centra of the vertebrae 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 become 

 bony, but is not replaced by membrane 

 bones, unless perhaps in Pterodactyls. 



When there is an interclavicle, it remains 

 distinct from the clavicle and sternum. 



The hand has more than three digits, 

 and at least the three radials 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 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 posterior body. 



The body has approximately the tem- 

 perature of the surrounding medium. 



The optic lobes lie on the upper surface 

 of the brain. 



BIRDS. 



There is an outer covering of feathers, 

 and though there may be a few scales, 

 there are never scutes. 



The centra of the vertebrae have usually 

 a peculiar terminal curvature. 



The two sacral vertebrae have no ex- 

 panded ribs, they fuse with others to form 

 a long composite sacrum. 



The cartilaginous sternum is replaced 

 by membrane bones 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 radials are clawed. 

 The fore-limbs are modified as wings ; 

 some carpals fuse with the metacarpals. 



The ilia are greatly prolonged in front 

 of the acetabulum, the inner wall of which 

 is membranous. The pubes slope back- 

 wards, parallel with the ischia ; only in 

 Struthio is there a pubic symphysis, only 

 in Rhea is there is an ischiac one. 



There are not more than four toes ; the 

 proximal tarsals unite with the tibia, 

 forming a tibio-tarsus ; the first meta- 

 tarsal 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 very high. 



The optic lobes lie on the side 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. 



