104 niGU SCHOOL ZOOLOGY. 



that rctunied from the body, but tliero is also a tendency 

 towards the sub-division of the ventricle, so that the blood 

 which has been aerated is kept towards one side of that 

 chamber, and sent chiefly towards the head. 



6. In an important respect the Snapping Turtle resembles 

 the other fSauropsida and diffei-s from most of the Ichthyopsida, 

 viz., the large size of the eggs, which is due to the large quantity 

 of food-yolk present. The eggs are like a bird's, except for the 

 less calcareous shell and the scantier white, and the embiyo i? 

 gradually developed in it at the expense of the yolk. The 

 period of oviposition is June, some twenty or thirty eggs of 

 the size of a pigeon's being then laid by the mother in a hole 

 scraped out by the hind feet, and not far from a stream or 

 pond. The sun's rays beating on the sandy soil generally 

 selected offer the requisite amount of heat for hatching the 

 eggs out about October. 



7. Chelydra is the type of a family named from it which 

 occupies a central position in the order of the Chelonia, and it 

 is easy to proceed from it to the wholly aquatic turtles on the 

 one hand, and the wholly tem-estrial forms on the other. At the 

 two extremes are forms which differ very materially from each 

 other in the adaptation of the form of the body to the sur- 

 roundings. The Marine turtles are very much depi'essed, their 

 feet are converted into flippers, and the carapace is not adapted 

 for the protection of the retracted head and limbs ; on the other 

 hand, the purely teiTestrial forms have a very convex carapace 

 within which the head, tail, and limbs can be sheltered, and in 

 some forms (the box-turtles) the plastron is hinged in such a 

 way as to close effectually the anterior and posterior apertures 

 into the shell. 



8. We may first proceed in the direction of the more aquatic forms, 

 of which the soft-shelled turtles, TrionycMdse, are fresh-water animals. 

 There are two coinmon species Amy da 7nutica and Aspidonectes sp'mifer, 

 abundant in streams opening into the great lakes from the South, and in 



