THE CHELONIA, 1013 
Dermochelys, as the evidence now seems conclusive that that type 
has originated in Tertiary times from a Cryptodeiran ancestor. 
Inall Chelonia the trunk is very markedly separated from the 
neck and tail, being very broad and completely surrounded by 
the shell. The shell of all Chelonia in which it is not reduced 
has the following structure :— 
There is a median dorsal row of dermal plates. There is 
always a nuchal, sometimes a postnuchal, then eight or fewer 
neurals and one or two pygals. The whole of this series seems to 
be homologous, but the eight neurals are distinguished from the 
other meen Glemients by their definite enmalesion with the 
neural spines of the dorsal vertebra. It seems certain that at 
one time the nuchal ete. were also directly associated with the 
anterior and posterior dorsal and sacral vertebra, and that they 
lost this primitive connection after the formation of the shell, 
either by its exaggerated growth or by a shortening of these 
regions of the vertebral column, so as to allow of the retraction 
of the head and limbs within the shell. 
All Chelonia have a row of costal plates on each side of the 
neurals ; these are invariably eight in number and are fused with 
the dorsal ribs from the second to the ninth. The development 
of these plates shows that they are of purely dermal origin, and 
they may be directly compared with the paired scutes found in 
Crocodiles and many allied reptiles. 
The fact that they have only fused with eight of the ten dorsal 
vertebre shows that these segments in ‘‘ Archichelone ” were in 
some way specialised and distinguished from the first and last 
dorsals. 
The marginal elements of the shell are not correlated with the 
ribs, and perhaps represent a different order of scutes as is 
suggested by Versluys. 
The plastron of Cryptodeires and some Pleurodeires consists of 
a single median and four pairs of plates. In most Pleurodeires 
and Amphichelydia an extra pair of plates is inserted, and in the 
remarkable Triassic Protochersis recently described by Fraas 
two extra pairs. 
The median and the anterior pair of plastrals are universally 
recognised as the interclavicle and clavicles. The other pairs are 
usually homologised with ventral ribs: that they resemble the 
latter in being ventral dermal ossifications is of course true, but 
I do not think it at all follows that they are necessarily derived 
from them. 
There is, for example, never any trace of a median or other 
lateral rows in the plastron of Chelonia, whilst abdominal ribs 
usually have an angulated median and more than one lateral row 
of elements. 
Furthermore, as “ Archichelone ” certainly had a series of dorsal 
scutes comparable to those of a crocodile, it is by no means 
unlikely that it resembled many members of that group in having g 
ventral scutes not homologous with abdominal ribs. 
