chap, xvi Backboned Animals 263 



A few snakes have months which do not distend, skull bones 

 which are slightly movable, teeth in one jaw (upper or lower) 

 only, and rudiments of hind legs. These are included in the 

 genera Typhlops and Anomalepsis, and are small simple ophidians. 



Many are likewise non-venomous snakes, but with wider gape 

 and more mobile skull bones, and with simple teeth on both jaws. 

 Some are very large and have great powers of strangling. Such 

 are the Pythons, the Boa, and the Anaconda. To these our grass 

 snake ( Tropidonotus natrix) is allied. 



Many poisonous snakes have large permanentlyerect grooved fangs 

 in the upper jaw, and a salivary gland whose secretion is venomous. 

 Such are the cobra (Naja tripudians), the Egyptian asp (Naja haje), 

 the coral snakes (Elaps), and the sea snakes (Hydrophis). 



Other poisonous snakes have perforated fang teeth, which can 

 be raised and depressed. Such are the vipers ( Vipera), the British 

 adder (Pelias berus), the copperhead (Ancistrodon contortrix), the 

 rattlesnakes ( Crotalus). 



Tortoises antl Turtles (Chelonia). — Boxed in by a bony, 

 shield above and by a bony shield below, and often with partially 

 retractile head and tail and legs, the Chelonians are thoroughly 

 armoured. On the average the pitch of their life is low, but their 

 tenacity of life is great. Slow in growth, slow in movement, slow 

 even in reproduction are many of them, and they can endure long 

 fasting. It is said that a tortoise walked at least 200 yards, twenty- 

 four hours after it was decapitated, while it is well known that the 

 heart of a tortoise will beat for two or three days after it has been 

 isolated from the animal. In connection with their sluggishness it 

 is significant that the ribs which help to some extent in the respira- 

 tory movements of higher animals are soldered into the dorsal 

 shield, thus sluggish respiration may be in part the cause, as it is 

 in part the result, of constitutional passivity. All the Chelonians 

 lay eggs in nests scooped in the earth or sand. 



The marine turtles (e.g. Sphargis, Chelone), the estuarine soft- 

 shelled turtles (e.g. Aspidonectes), the freshwater turtles (e.g. 

 Emys), and the snapping turtle (Chelydra) are more active than the 

 land tortoises, such as the European Testudo grtzca, often kept as 

 a pet. The tortoise of the Galapagos Islands ( Testudo elephantopus), 

 the river tortoise (Podocnemys expansa) of the Amazon, the bearded 

 South American turtle (Chelys matamata), and the green turtle 

 (Chelone mydas) attain a large size, sometimes measuring about 

 3 feet in length. 



Crocodilians (Crocodilia). — Crocodiles, alligators, and gavials 

 seem in our present perspective very much • alike — strong, large, 

 heavily armoured reptiles, at home in tropical rivers, but clumsy 

 and stiff-necked on land, feeding on fishes and small mammals, 



