96 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



afforded us a complete series of intermediate stages 

 between the reptiles and birds, and they are justly 

 united in the common group of the Sauropsida. 



A. Reptilia. Sauroids with horny or bony 

 plates, but no feathers, with more than three digits in 

 the manus, of which three at least bear claws, with at 

 least three digits in the pes, and with unankylosed 

 metatarsals. The blood is ordinarily cold, and there 

 is at least one pair of aortic arches. 



1, 2. Lacertilia, or lizards, and Op hi ilia, or 

 snakes, have the quadrate movable, the penis double, 

 and the anus a transverse slit. Some of the Lacer- 

 tilia, such as Lacerta (the common lizard), are 

 the least modified of all Sauroids, and the Geckos 

 retain a primitive character in the persistence of 

 remnants of the notochord. Others are specially 

 modified, like the flying lizard (Draco), others have 

 ossified scutes approaching those of crocodiles (e.g. 

 Cycled us) ; Hatteria is remarkable for the possession 

 of " uncinate processes " on the ribs (see page 346), 

 such as are seen in crocodiles and birds. Some, like 

 the blind-worm, lose their limbs, but all have a pectoral 

 arch and a urinary bladder, both of which are absent 

 from the Opliiclia, in which the hind limbs are rarely 

 present, and then are only short and inconspicuous. 

 They are divisible into the Eurystomata, in which 

 the mouth-cavity is capable of dilatation, and the 

 Stenostomata, in which the facial bones are im- 

 movably connected with one another. Among the 

 former we find vipers, rattlesnakes, and water snakes, 

 which are venomous ; and adders, boas, and pythons 

 which are not so. Typhlops and Uropeltis are 

 examples of the Stenostomata. 



3. Chelonia, or turtles and tortoises. In these 

 the quadrate is immovably connected with the side 

 of the skull, the penis is simple and solid, and the 

 anal orifice rounded. The bony plates developed in 



