enclosing shields. The upper shield is called the carapace, 

 the lower the plastron. The carapace is formed by the 

 coalescing of the vertebrae of the back with the much ex- 

 panded and suturally united ribs, 1 and usually by an addi- 

 tional series of dermal marginal bones ; this whole shield 

 is overlaid by epidermal scales or plates. The plastron is 

 formed by eight to eleven dermal bones which do not rep- 

 resent a true sternum ; this shield, too, is covered by epi- 

 dermal plates. In neither shield do the covering plates 

 agree in size or arrangement with the underlying bones. 

 Both shields are united in varying degree at the sides. 



The only parts of the spinal column independently mo- 

 bile are the neck and tail. There are always eight cervi- 

 cal, twelve dorsal and two sacral vertebrae ; the caudal 

 vertebrae vary in number, but they are never numerous. 

 The pelvic bones are not attached to a sacrum. The limbs 

 are four, which sometimes are paddle-like. 



The toothless jaws are provided with horny cutting 

 sheaths. The skull is massive, the bones united by 

 sutures ; a quadrate is present. There are eyelids and 

 nictitating membrane ; also a tympanum. The vent is 

 never a cross-slit. The limbs, head and tail are covered 

 with naked skin, usually more or less provided with scales 

 or tubercles. 



The earliest remains of turtles are found in the Triassic 

 formations. 



Upwards of 220 living species are now known. 



The eastern United States are quite rich in species and 

 bear much resemblance in several respects to the Indian 

 region. 



Turtles usually have been subdivided according to their 

 habitat, into marine, fluviatile, paludine and terrestrial. 



Of the true land turtles, more properly called tortoises, 

 none are found in the Northern States. Our local specie^ 

 (including those of doubtful occurrence) we will now con- 

 sider individually. 



1 Except in the Dermochelydidce, which see. 



