CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
157 
analogy of the thorax of the Bird : but they are not elucidated by any special de- 
Fig. 4. 
Thoracic segment, Tortoise. 
scriptions or figures. They will be, per- 
haps, best understood by comparing the 
subjoined view of a segment of the 
thoracic-abdominal case of the Tortoise 
(fig. 4) with the corresponding view of 
the homologous segment in the Bird 
(fig. 5) ; in both of which c is the verte- 
bral body or ^centrum,’ n the neural 
arch, m the neural spine, pi is the ver- 
tebral rib (pleurapophysis), h (the outer 
letter in fig. 4) is the ossified sternal rib 
(heemapophysis), and hs the haemal spine 
or ‘ sternum.’ In this comparison it is 
supposed that the primitive median divi- 
sion of the sternum is retained in the 
cold-blooded reptile, and that the keel, 
or ‘ entosternal ’ piece {hs'), continues 
distinct, but is developed in breadth in- 
stead of depth. No one, however, has 
been able to adduce any example from 
the class of birds in which the lateral 
moieties of the broad sternum are deve- 
loped each from four distinct centres, 
answering to the four lateral or parial 
pieces in the plastron of the Chelonia {es, 
hs, ps and xs, fig. 3). 
The homologies of the carapace and 
plastron, regarded as developments of 
the endo-skeleton, appear, hitherto, not 
to have been elucidated by any other 
comparison, save that by Carus with the 
thorax of the Frog. Yet the chelonians 
have nearer affinities to the crocodiles 
than to either birds or batrachians ; and 
a comparison of the thoracic-abdominal 
part of the skeleton of a crocodile ap- 
pears to give correspondingly closer illus- 
trations of the nature of the peculiarities 
of that in the Tortoise. , In the sub- 
joined view of the segment of the thorax of a crocodile (fig. 6), it will be observed 
7» s* 
Thoracic segment, Bird. 
Fig. 6. 
Thoracic segment, Crocodile. 
