Chap. I. THE SKELETON. 2G5 



tli.at all the nine sternal bone.s of the Turtles are not mere dermal ossifications, as 

 Rathkc,^ misled by the attachment of the muscles inside, would suppose, but that they 

 really belong to the skeleton, being regular cartilages with distinct forms, and of the 

 same shape as the bones in the adult. In the same way we have ascertained that 

 the marginal bones are mere ossifications of the .skin, and by no means to be com- 

 jmred with the long bridges which connect the true ribs and the sternum in Birds, as 

 GeofFroy, and after him, Dumeril and Bibron, believed.^ We found, farther, that 

 that strange crosspiece, the foremost transverse bone in the carapace, is a regular 

 skeleton bone, though I do not venture to call it either a rib or a transverse 

 process of the last nock vertebra, as one might perhaps think it to be. There 

 are limits to explaining and homologizing. We cannot make up a Bird from 

 the bones of a Turtle, nor a Man from the bones of a Fish, as some anatomists 

 have recently tried to do, who misunderstood the great thoughts of Oken and 

 other philosophers respecting the structure of the skeleton. 



If we go back to the earliest stages of growth of the Testudinata to ascer- 

 tain the true character of their bony shield, it will be easy to show that the 

 bony walls which, in the adults, form the dorsjd and pectoral shields, consist at 

 firet simply of cells, out of which the skeleton, the muscles, and the .skin are 

 formed in the end, in all Vertebrates, and that it is not the skin only which is 

 here absorbed into the skeleton, but the whole animal wall. This view of the 

 case may render more intelligible the apparently abnonnal position of the limb.s, 

 and the mode of attachment of the pectoral muscles. 



SECTION VI. 



THE SKELETON. 



He(((I. The skull in the Turtles is more solid and compact than in the Saurians 

 and Ophidians; the bones of the dice, in particular, are immovably fixed to the 

 skull-box; the os quadratum is also soldered to it by a tight suture as in Crocodiles 

 and in Mammalia, while in the other Reptiles and in Birds it is jointed to the 

 skull only by ligaments and a socket. The lower jaw is fonned of one solid, 

 bony arch, the soft symphysis between its branches having entirely disappeared 

 as in Bird.s, while in Saurians this symphysis alwaAS remains more or less carti- 



^ See Ralhke, Ucbur tlic Entwiikc'lung ilcr .Si-liild- ^ Gcoffroy, in Aiinalos du Museum, vol. xiv. 



kroten, p. 122. Dumeril luul Bibron, in Erpt-tologie generale, vol. i. 



34 



