204 VERTEBRATES. 



3. That the spinal column is coossified with the cara- 

 pace, only the vertebrae of neck and tail remaining free 

 and movable. 



I. The Shell : Exoskeletal Parts. Examine the plas- 

 tron. On its ventral surface find six pairs of flat, horny 

 plates, with two additional smaller pairs, forming the 

 margins of the bridges meeting the carapace. On its 

 dorsal or inner surface find four pairs of plates and one 

 single plate of bone. Note that the boundaries of the 

 horny plates on the ventral surface do not correspond 

 to the boundaries of the bony plates on the dorsal sur- 

 face. The horny plates, like the scales of other parts, 

 are developed from the outer, epidermal layer of the skin, 

 and are therefore called epidermal plates. The bony 

 plates are developed from the deeper layer of the skin 

 (dermis), and are called dermal plates. 



Examine the carapace. Its convex surface is com- 

 pletely covered over with epidermal plates, as follows : 



On the median line, at the anterior edge, is a small, 

 single nuchal plate (sometimes a pair of nuchal plates). 



Posterior to this, on the median line, are five large 

 central plates. 



A pair of pygal plates meet on the median line, at the 

 posterior edge. 



The central plates are bordered laterally by four pairs 

 of large centrolateral plates. 



The margins of the shell between the nuchal and pygal 

 plates are formed by eleven pairs of marginal plates. 



Dermal plates are developed beneath the marginal 

 nuchal and pygal epidermal plates, and correspond with 

 them in position, in names, and in number, save that the 

 nuchal and pygal plates are single. 



The dermal and epidermal plates together constitute 

 the outside skeleton or exoskeleton, as distinguished from 

 the true internal skeleton (or endoskeleton). 



