548 ORDER CHELONIA. 



IV. BATRACHIA, or Frogs ; which have neither ribs nor 

 sternum, but which possess four members ; the skin is soft and 

 naked (or uncovered with scales) ; and in their early state they 

 respire by means of gills, which are sometimes retained through 

 the whole of life. 



As the chief peculiarities of these several orders have been 

 already noticed, a very short account of the principal families 

 included in them will suffice. 



ORDER I. CHELONIA. 



486. The animals composing this order vary considerably in 

 those details of their structure, which adapt them to different 

 habits of life ; some of them being destined to reside exclusively 

 upon the solid ground, others to pass nearly their whole time in 

 water, and others to dwell amidst marshes, the muddy banks 

 of rivers, &c. The most obvious variations are in the structure 

 of the feet, and in the form of the shell. Thus in the proper 

 Land-Tortoises, we meet with a short stumpy foot, not unlike 

 that of an Elephant, the toes not being separated, and only the 

 claws apparent (Fig. 326.) In the Marsh and River- Tortoises, 

 the toes are divided and webbed, so as to increase the extent of 

 surface; and in the Marine- Tortoises, or Turtles, we find them 

 extended into large undivided paddles, by which the animals 

 can propel themselves rapidly through the water (Fig. 328). 

 The carapace is highly arched and very strong in the Land-Tor- 

 toises (Fig. 326) ; but more flattened in the aquatic families, for 

 the better adaptation of their form to motion in a liquid. The 

 plastron differs considerably in degree of development, and in the 

 relative consolidation of its different parts. It is most complete 

 in the Land-Tortoises ; in many of which, the anterior and 

 posterior portions of it are so jointed to the centre-piece, as to 

 be able to close the orifices before and behind, after the head, 

 tail, and legs have been drawn in, thus affording to the animal 

 the most excellent means of passive defence. The natural food 

 of many of the Chelonia is vegetable, and they can endure long 



