CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
153 
and eleventh ^ neural plates,’ they have less claim than the first dorsal vertebra to be 
regarded as entering into the composition of the carapace. 
The ‘ plastron ’ (fig. 3) or floor of the thoracic-abdo- 
minal chamber consists in all Chelonia of nine pieces, 
for which the terms proposed by Geoffroy St. Hilaire 
may be retained, if used in an arbitrary sense and 
without implying assent to the hypothesis that first 
suggested them. The median and symmetrical piece 
of the plastron (fig. 3) is the ‘ entosternal’ {s), the four 
pairs, counted from before backwards, are respec- 
tively, the ^ episternals ’ {es), ‘ hyosternals ’ {hs), ‘ hy- 
posternals’ {ps), and xiphisternals ’ {xs). 
With regard to the ideas that have been entertained 
as to the homologies of the above-defined osseous 
pieces, it would be a parade of names without adequate 
gain to the discussion, to go further back than the first 
edition of the ‘‘Lecons d’Anatomie Comparee” (1799), in which Cuvier refers the 
chief part of the carapace, viz. the ‘costal plates,’ to eight pairs of dilated ribs*"; 
the neural plates he describes as corresponding in number with the vertebrie of which 
they form part'f-: the marginal pieces and the parts (“plusieurs os”) of the plas- 
tron are described arbitrarily and left undetermined. 
Geoffroy St. Hilaire, entering into the question of their homologies in his 
memoir on the genus Trionyx, published in the year 1809, and adopting the Cuvierian 
idea that the carapace consisted of a development of dorsal vertebrae and vertebral 
ribs, argues that the plastron is a greatly expanded sternum, and that the marginal 
pieces are the cartilages of the ribs ossified, or ‘ sternal ribs ’ (“ cotes sternales:|: ”). 
In the collection of memoirs forming the first edition of the ‘Ossemens Fossiles,’ 
Cuvier merely cites the opinion of Geoffroy, “ Ces pieces que M. Geoffroy com- 
pare a la partie sternale ou cartilagineuse de nos cotes manquent aux tortues molles," 
Fig. 3. 
* “La carapace des Tortues est formee par les dilatations de huit cotes ou batons osseux qui prennent nais- 
sance sur les unions des vertebres et se terminent a un rebord que entourent toute la carapace.” Tom. i. p. 211. 
t “ On remarque en dessus, le long de la partie moyenne, une rangee de petites plaques osseuses joresque 
carrees, unies intimement entre elles par synarthrose, qui sont en meme nombre que les vertebres dont elles font 
partie.” Ib. p. 211. 
+ “ La difference dans le nombre des pieces du plastron et du sternum des oiseaux pouroit faire croire qu’il 
seroit entre dans le plastron des tortues des pieces etrangeres a la composition d’un sternum, comme des cotes 
sternales ; idee d’autant plus naturelle a admettre, que les parties laterales du plastron sont terminees par un 
certain nombre de digitations ; cependant il n’en est rien. Les analogues des cotes sternales ne manquent 
point dans les tortues; elles existent dans ces pibces articulees dont j’ai parle plus haut, et se voient a la suite 
des cotes vertebrates ou elles forment le bord des carapaces. Le plastron, ou le sternum des tortues s’attache 
sur ces cotes ou pieces sternales, en sorte qu’il ne manque rien d’essentiel dans le thorax des ces animaux.” — 
Annales du Museum, xiv. p. 7 (1809). 
MDCCCXLIX. X 
