ANATOMY OF VERTEBRATES. 



G3 



53 



ginal scutes,' supported by the marginal plates, and crossing their 

 sutures. 



In the TrionycidcB the exterior surface of the carapace and 

 plastron is remarkable for its rough vermicular or punctate 

 sculpturing. 



The median bony pieces of the carapace, fig. 52, ch, s 1 to s n, 

 have been regarded as lateral expansions of the summits of the 

 neural spines ; the medio-lateral pieces, ib. pi l to pi 8, as similar 

 developements of the ribs ; and the marginal pieces ib. m\ to m 13, 

 as the homologues of the sternal ribs. But the developement of 

 the carapace shows that ossification begins independently in a 

 fibro-cartilaginous matrix of the corium in the first, ch, and some 

 of the last, s 9 to s n, median plates, 

 and extends from the summits of the 

 neural spines into only eight of the in- 

 tervening plates, s l to s 8 : ossification 

 also extends into the contiguous lateral 

 plates, pi l to pi 8, in some Chelonia, 

 not from the corresponding part of the 

 subjacent ribs, but from points alter- 

 nately nearer and farther from their 

 heads, 1 showing that such extension of 

 ossification into the corium is not a 

 developement of the tubercle of the 

 rib, as has been supposed. Ossification 

 commences independently in the corium 

 for all the marginal plates, m l to py ; these never coalesce with the 

 bones uniting the sternum with the vertebral ribs, are often more 

 numerous, sometimes less numerous, than those ribs, and in a few 

 species are wanting. Whence it is to be inferred that the ex- 

 panded 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. Along the under surface 

 of the costal plate the slender or proper portion of the rib may 

 be traced, of its ordinary breadth to near the head, which liberates 

 itself from the costal plate, as at I, fig. 51, to articulate to the in- 

 terspace of the two contiguous vertebra?, to the posterior of which 

 such rib properly belongs. 



In the 'plastron,' fig. 53, the entosternal, s, answers to the 

 sternum in the crocodile : the parial pieces are ' hoemapophyses ' or 



Plastron of Clielone caouanna 



1 clxii. p. 163, pi. xiii. fig. 4. 



