CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
165 
plates, being the formation of a strong defensive abode, although the existence of the 
cuticular scutes, rather than of the parts of the endo-skeleton, determines the com- 
mencement of the ossification in the Tortoises, yet such ossification begins and pro- 
ceeds in the dorsal and sternal integument of those Chelonia, e. g. Trionyx and Sphar- 
gis, that have no cuticular scutella, but a soft, uniform and lubricous integument. 
The influence, however, of this modification of the cuticular system on parts regarded 
as homologous with endo-skeletal elements in the ordinary Chelonia, is strikingly 
manifested, in the condition of the marginal plates and the variable proportions and 
even in the number of the ossified parts of the plastron, as e.g. in that of the Trionyx 
granosus {Cryptopus, D. & B.), in which a single dermal bony plate extends over 
the rudimentary heemapophyses called hyosternals and hyposternals on each side. 
The cartilaginous matrix in the substance of the qorium forming the margin of the 
carapace of Sphargis and Trionyx, receiving no stimulus from the presence of mar- 
ginal scutella, is found to contain either mere scattered granules of ossific matter, 
as, €. g. in the Trionyces forming the genus Gymnopus of Dumeril and Bibron ; or 
centres of ossification are established, as at the posterior part of the limb of the cara- 
pace in the species of Cryptopus, D. & B., which have no relation whatever with the 
presence, number or position of the vertebral ribs ; and in these conditions of the 
border of the carapace we perceive a greater or less retention of the embryonic cha- 
racter noticed in the genus Chelone. 
Summary. — The conclusions as to the homologies of the Chelonian carapace and 
plastron to which I have arrived from the observations above recorded and other de- 
tails with which it has not been deemed necessary to encumber this communication, 
are as follow : — 
1st. The centrum and the neural arch supporting the neural plate are parts the 
homologies of which admit of no question, and have given rise to none ; but the 
neural plate itself is a dermal bone homologous with the median dermal scutes of 
the Crocodile’s back-shield, but connate in some of the dorsal segments with the true 
neural spine in the Chelonia. 
2nd. Only the free proximal and distal extremities of the costal plate and the nar- 
row smooth prominent tract * continued from the one end to the other along the 
under part of the plate, represent the ‘ pleurapophysis ’ or vertebral rib; and this 
rib is accordingly simple, as in other reptiles with a tripartite heart : the external 
expanded portion which joins the neural plate is a dermal bone homologous with 
the medio-lateral dermal scutes in the Crocodiles, but connate with the pleurapo- 
physis in the Chelonia. 
3rd. The marginal plates are wholly dermal scutes; and even on the hypothesis 
that any of them, such for example as are penetrated by the free ends of the pleur- 
apophyses, belong to the endo-skeleton, yet these answer rather to the accessory in- 
* This part is well shown in the view of the inner surface of the carapace of the ‘ Water-Tortoise,’ which 
Cheselden gives in the beautiful plate facing the 3fd Chapter of his magnificent ' Osteographia,’ fol. 1733. 
