168 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
stronger evidence, to suppose such defects in his description. They are, however, 
plainly demonstrated in my preparations, and are accurately represented in the 
figures 4 and 5 of Plate XIII. I entirely concur with Prof. Rathke in regarding 
the marginal plates as dermal bones : in every particular of developmental character 
they agree with one-fourth of the median series of bony plates of the carapace (neural 
plates); and in every respect, save connation with endo-skeletal bones, they agree 
with the rest of the median series and with all the costal series of bony plates of the 
f 
carapace. 
Finally, there remains for consideration Prof. Rathke’s peculiar hypothesis of the 
nature of the plastron. 
“ The development of the plastron,” he says, “ takes place later than in Birds and 
Mammals : the cartilaginous basis consists of two pairs of very narrow and thin car- 
tilages, one in front and the other behind the umbilical aperture : there is likewise 
formed a fifth azygous piece in most Chelonians” (he excepts with a doubt), 
“ between the two anterior parial pieces .... Subsequently there are developed in 
these different cartilages, more numerous osseous pieces, ordinarily, or perhaps always, 
nine in number. Their respective size varies greatly, according to the different spe- 
cies of Chelonians ; for either they grow in such a way, the one in front of the other, 
that they meet by their corresponding borders in their whole length, so as to consti- 
tute a continuous plastron, or their growth is more restricted, and then they form a 
plastron open in the middle ; or they constitute merely a narrow ring, as is probably 
the case in the Sphargis*." 
He alludes to other modifications of growth, which might equally have been sug- 
gested by the known varieties of the plastron in the adults of the different genera and 
species of Chelonia ; e.g. where it extends forwards beyond the neck, and backwards 
beyond the tail, which he thinks may probably depend upon the presence of an infe- 
rior fold of chorion existing in front of the fore-feet, and of another inferior fold be- 
hind the hind-feet : although he admits that where, as in the Trionyx, such folds 
occur, they are not occupied by the plastron, which fact invalidates the hypothesis. 
My observations do not agree with those of Rathke, which have led him to ascribe 
the eight parial pieces of the plastron to the development of as numerous osseous 
pieces in the two pairs of primitive slender cartilages. I find no other ossification 
set up on the anterior pair of those cartilages than that which results in the forma- 
tion of the hyosternals ; and no other in the posterior pair than that which results in 
the formation of the hyposternals. The episternals unquestionably have independent 
cartilages, and so I believe have the xiphisternals, though I have failed to get so 
clear a demonstration of the latter. 
The primitive cartilages of the true sternum (entosternal) and the thoracic-abdo- 
minal hsemapophyses (hyosternals and hyposternals) are distinct from, and deeper- 
seated than, the thin stratum of cartilage-cells which pervades and thickens the ven- 
tral fibrous integument. I am unwilling to suppose that Rathke could have ever wit- 
* Loc. cit. 
