166 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
tervening pieces between the pleurapophyses and haeniapophyses of the Crocodilian 
thorax* * * § , than to the hsemapophyses in their totality. 
4th. The parial or lateral parts of the plastron, more especially the hyosternals and 
hyposternals, are the true ‘ hsemapophyses’; but in connation with dermal bony plates 
to which their characteristic breadth, especially in the land and freshwater Chelo- 
nians, is chiefly due. The entosternal, and perhaps the episternals, which repeat the 
transverse bar of the T-shaped sternum in Lacertia and Monotremata, are the sole 
parts of the plastron which can be referred to the ‘ sternum ’ in special homology and 
to the ‘ hsemal spine ’ of the typical vertebra in general homology. 
Supplement. 
The justly-merited reputation of Prof. Rathke as an embryologist, and the fact 
of his having deduced his views of the mixed nature of the thoracic-abdominal part 
of the skeleton of the Chelonian Reptiles from observation of its development, equally 
demand that his conclusions should not be abandoned without special grounds being- 
assigned. Rathke concludes, as has been before stated, that the carapace belongs to 
the endo-skeleton exclusively'!', and the plastron as exclusively to the exo-skeleton 
With regard to the carapace, he says, “ The spinons processes are already deve- 
loped from the second to the eighth dorsal vertebr 2 e before the exclusion of the em- 
bryo, they remain pretty short, but contrary to the general laws of development 
of the vertebrate animals, they grow so much in breadth, that they form, after their 
ossification, a series of horizontal plates of moderate size§.” He also takes occasion 
to confute the assertions of Carus, Wagner and Peters, that these plates are first 
developed independently in the derm and afterwards coalesce with the spines of the 
subjacent vertebrae. My observations concur with those of Rathke in regard to the 
fact that the neural plates, answering to the eight vertebrae of the carapace, are not 
developed independently of the neural spines, but are connate with, or ossified con- 
tinuously from them []. Nevertheless the position of the pre-existing fibro-cartilaginous 
matrix, and the distinctive character of the resulting ossification, appear to me to be 
stronger grounds for determining their dermal homology, than the mere fact of their 
connation in opposition to that view. The radius and ulna of the Frog are not only 
confluent but connate ; i. e. they are progressively or continuously ossified from a 
* These are well shown by Cheselden in the side view of the skeleton of the Crocodile, which forms the 
vignette of chapter hi,, op. cit. They are not noticed in either of the editions of Cuvier’s ‘ Ossemens Fossiles,’ 
or of the ‘ Lecons d’Anatomie Comparde,’ and are therefore unnoticed in most of our English compilations of 
Comparative Anatomy. 
t Ueber die Entwickelung der Schildkroten. 4to, p. 105. t Ik. p. 122—129. 
§ Loc. cit., and Annales des Sciences Naturelles, Mars, 1846. 
11 At least in the Testudo and Chelone. In some Trionyces ossification extends into the eighth neural plate 
from the median ends of the eighth costal plates, and in a new species which I have called Trionyx planus the 
same modification supersedes the seventh neural plate. These varieties are very significative of the dermal cha- 
racter of the neural plates. 
