CHELONIANS. 459 



therefore, it follows that an egg requires four years from the 

 time there exists a marked difference among the eggs of different 

 sizes, to acquire its full maturity."* Fresh-water Tortoises lay 

 their eggs in moist ground, or in dryer places near the water; 

 Marine Turtles lay theirs in hot sand ; the Land Tortoises lay 

 theirs upon dry ground. The time of the extrusion of the ani- 

 mal varies from six weeks to three or four months. 



The divisions of the Chelonians vary, as made by different 

 naturalists. Agassiz, who prefers names which have priority of 

 dale, divides the order TESTUDINATA, or CHELONIA, into two sub- 

 orders. 



I. AMYDJ2, with seven families. 



(1) Testudinina,) Land Tortoises. 



(2) Emydoida, (3) Cinosternoida, ) R . , M , 

 (4) Chelydroid*, (5) Hydraspida, > ToSfoea 

 (6) Chelyoida, (7) TrwnycUda, ) 



II. CHELONII, with two families. 



(1) Chelonioida, (2) Sphargidcz, Marine Turtles. 



FIRST SUB-ORDER. AMYD.E. DIGITATED. 



This sub-order includes, besides the Land Tortoises, the 

 Marsh and River Tortoises, intermediate in form between the 

 Marine and Land Tortoises. The Marsh Tortoises proper, are 

 sometimes arranged into two divisions, viz: (1) the Pleurodera, 

 (Gr. pleuron, a side ; deire, neck,) so named because the head 

 is concealed, not by being drawn back in a straight line, but by the 

 neck folded to one side of the opening of the shell. ,. The arms 

 are also incapable of being completely drawn within the cara- 

 pace and plastron. Seven is said to be the number of the genera 

 included in this division, None of the species belong to North 

 America, but many to South America. (2) The Cryptodera, 

 (Gr. krupto, to conceal ; deire, neck,) in which the cylindrical 

 shaped neck can be folded back upon itself under the center of 

 the forepart of the carapace. The pelvis is articulated to the 

 internal surface of the carapace, while in respect to the plas- 

 tron, it is free ; this gives to many of the Marsh qr Pond Tor- 

 toises a power to .move the under portions of the osseous box ; 

 and this, accordingly, has a less degree of solidity ; whereas, in 

 the Pleurodera, the pelvis is firmly fixed to the roof of the cara- 

 pace above, and to that of the plastron beneath. The species of 

 this division are very numerous, and many are found both in 

 North and South America. 



* Contributions to the Natural History of the United States. Vol. I. 



