300 A COURSE ON ZOOLOGY. 



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piration of the tortoise very different from that of ani- 

 mals whose ribs are mobile, and tortoises take in air by 

 an act of swallowing. This air enters the mouth by the 

 nostrils, and the tongue acts as a valve, which forces it 

 into the pharynx and towards the lungs. 



According to the form and habits, the chelonia may 

 be arranged in three groups : sea tortoises, fresh-water 

 tortoises, and land tortoises, The aquatic species are 

 generally called turtles. 



The land tortoises have large feet, terminating in a sort 

 of stump, and usually a very arched carapace, which com- 

 pletely protects all parts of the animal. The largest 

 American species is the gopher tortoise, found in the 

 Southern States ; its length is about fourteen inches, its 

 shell very convex, brownish yellow, tinged with dark 

 brown. The box tortoise is our common land tortoise, 

 and grows. to a length of about six inches. The Greek 

 tortoise, found on the borders of the Mediterranean, is 

 much esteemed as food. All the land tortoises bury 

 themselves in the soil during the winter, sleeping there 

 until spring. 



The fresh-water turtles have palmate feet and a flat- 

 tened carapace, sometimes without plates and covered 

 with a soft skin. One of the smaller species is. the mud- 

 turtle, about six inches long ; it is found all over North 

 America ; the skin of its limbs is orange-colored, and it 

 is edible. Larger species are the alligator terrapin, 

 found in Southern waters, and the snapping-turtle, well 

 known for its strength and ferocity. 



The tide-water turtles are usually called terrapin, and 

 are much sought for their flesh ; among them the dia- 

 mond-back, which attains a length of six or seven inches, 

 is most prized. 



