CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
169 
nessed the marked distinction between the ossification of these endo-skeletal parts and 
the superadded dermal ossified layer, and have made no mention of it. He, in fact, 
maintains his opinion, that the plastron is nothing else but a part of the dermo- 
skeleton, and that it has nothing in common, in an anatomical point of view, with 
the sternum of other animals*,’’ fl^us diverging to an opposite extreme from that of 
the GeoflTroyan hypothesis, although rather by arguments drawn from the relative 
position of other parts of the skeleton and from Comparative Anatomy than from the 
actual phenomena of the development of the plastron. 
If the plastron of the Chelonia were the homologue of the sternum in other 
Vertebrates,” says Rathke, one must also admit that the bones composing the 
shoulder and pelvis are situated in a manner entirely contrary to the general dispo- 
sition of those parts in other animals.” But that remark would equally apply as an 
argument against the carapace being homologous with the vertebrae and ribs, as 
Rathke contends it to be. It appears to me, however, that the peculiarly advanced 
position of the scapular arch in the embryo Chelonia, and, at its first appearance in 
all other Vertebrates, in relation to the thoracic haemal arches, — a transitory relative 
position so beautifully explained by the recognition of the scapular arch as the haemal 
arch of the occipital vertebra — equally explains and removes the anomaly of its posi- 
tion in regard to the plastron of the adult Chelonians as in regard to the carapace. 
In both instances the Chelonian peculiarity or anomaly, in the relative position of 
the bones of the shoulder, is due to the retrogradation of the scapular arch and the 
concomitant expansion of certain succeeding haemal arches ; as, for example, that 
formed by the second pair of dorsal ribs above, and by the episternal and hyosternal 
bones below ; the one extending above the arch as a roof, the other beneath it as a 
floor. The discordance of the relations of the scapular and pelvic arches of the 
Chelonians with those in other Vertebrates no more necessitates the assumption that 
all the plastron belongs to the dermo-skeleton, than that all the carapace does. 
With regard, indeed, to the relations of the pelvis to the plastron, whilst we should 
look amongst other Vertebrates, in vain, for instances in which the ossified exo- 
skeleton is developed beneath it, as Rathke supposes it to be in the Chelonia (fig. 8, 
in which ps and xs are referred by that author to the exo-skeleton-f-), we have not far 
* Loc. cit. 
t In the figures 8 and 9 hh are the hsemapophyses or abdominal ribs, specified in the Chelonia as hs hyoster- 
nals, hypostemals, and xs xiphisternals : 62 is the modified pleurapophysis called ‘ilium’; 63 and 64, the 
modified haemapophyses, called respectively ‘ ischium ’ and ‘ pubis ’ ; 65, femur; 66, tibia; 67, fibula; 68, tar- 
sus ; 69, metatarsus and phalanges. 
In a fossil iJmys from Sheppey, described by Professor Bell, Sec.R.S., in our joint Monograph on the Fossil 
Chelonia of the London Clay, an intercalated piece is wedged in between the outer part of the interspace of 
the hyosternal and hyposternal on each side, like the dismemberments of the abdominal ribs at the outer part 
of that group of bones in the Plesiosaurus : and in another fossil Emydian from the same formation and loca- 
lity, an intercalated bony piece extends across between the hyosternal and hyposternal on each side of the 
plastron. See Description of Platemys Bullockii, in my Report on British Fossil Reptiles, Report of the British 
Association, 1841, p. 164. 
MDCCCXLIX. Z 
