CARAPACE OF THE TURTLE. 125 



and extends from tlie summits of the neural spines into 

 only eight of the intervening plates, s 1 to s 8 ; ossification 

 also extends into the contiguous lateral plates, i^ll to plS^ 

 in some chelonia, not fi'om the corresponding part of the 

 subjacent ribs, but from points alternately nearer and 

 further from their heads, showing that such extension of 

 ossification into the corium is not a development of the 

 tubercle of the rib, as has been supposed. Ossification 

 commences independently in the corium in all the margi- 

 nal plates, m 1 to py^ which never coalesce with the bones 

 uniting the sternum with the vertebral ribs, and which 

 are often more numerous, and sometimes less numerous 

 than those ribs, and in a few species are wanting. 

 Whence it is to be inferred that the expanded bones of 

 the carapace, which are supported and impressed by the 

 thick epidermal scutes called "tortoise-shell," are dermal 

 ossifications, homologous with those which support the 

 nuchal and dorsal epidermal scutes in the crocodile. 

 Most of the pieces of the carapace being directly con- 

 tinuous or connate with the obvious elements of the ver- 

 tebrae, which have been supposed exclusively to form 

 them by their unusual expansion, the median ones, si to 

 sll, have been called "neural plates," and the medio- 

 lateral pieces, pll to ^j)?8, "costal-plates;" but the exter- 

 nal lateral pieces, ml to ml2, have retained the name oS 

 " marginal plates." The first or anterior of the median 

 plates (c/i, "nuchal plate") is remarkable for its great 

 breadth in the turtles, and usually sends down a ridge 

 from the middle line of its under surface, which articu- 

 lates more or less directly with the summit of the neural 

 arch of the first dorsal vertebra ; the second neural plate 

 is much narrower, and is connate with the summit of the 

 neural spine of the second dorsal vertebra ; the seven suc- 



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