534 



MANUAL OF ZOOLOGY. 



Both the carapace and plastron are covered by a series of 

 horny plates (rarely wanting), which are developed in the 

 epidermis, and which are perfectly distinct from the bones 

 which they cover. As encasing the upper surface of the 

 carapace, these plates (which in some species constitute the 

 "tortoise-shell" of commerce) have a general arrangement 

 conforming with that of the bony plates beneath, though there 

 is no numerical correspondence between the two. Thus the 

 carapace, as we have seen, consists of (i) a median series of 

 " neural " plates developed from the vertebrae ; (2) a lateral 

 series of "costal" plates on each side, corresponding with 

 and largely formed by the ribs ; and (3) a peripheral series of 

 "marginal" plates (see fig. 293). Similarly, the epidermic 



Fig. 293. Carapace of the Loggerhead Turtle (Chelone caouanna), viewed from above 

 (after Owen). In this form, the ribs are separate and free towards their extremities, 

 and the osseous portions of the carapace are indicated by the light lines, while the 

 epidermic plates are marked out by dark lines, n n The first and last of the me- 

 dian series of "neural plates ;" c c The expanded ribs or "costal plates ;" m in 

 The first " marginal plate " on each side; mi Nuchal plate ; py Pygal plate; vv 

 Median series of epidermal plates, or " vertebral scutes." 



plates (fig. 293) are arranged in (i) a median, "vertebral," or 

 " neural " series ; (2) a lateral series on each side, of " costal " 



