264 AMERICAN TESTUDINATA. Part IL 



diiced by a mere enlargement and ovei'growing of the vertebrae and the rib,*!, that 

 is to say, by the peculiar development of certain bones of the true skeleton. 



The bony shield of Sphargis exhibits, moreover, some peculiarities which we do 

 not find in other Turtles. There is a most elegant pavement of small plates, 

 extending over the whole shield, seemingly jointed to each other by the finest 

 sutures, which, howevei*, are in fiict nothing but nutritive canals starting from those 

 seeming sutures, themselves larger canals, and ramifying through the plates as a 

 fine network of a yellow color, owing to the fat fluid which the canals contain. 

 As I possess no young specimens of this Turtle, I have had no chance to observe 

 the corium before it is ossified, so that this remains to be studied. The character of 

 the ossification is, however, really the same as in the demial ossification of Trionyx, 

 mentioned above, except that the canals seem to be more regular in Sphargis. 

 With reference to the extension of these ossifications, I have already made some 

 remarks above, when speaking of the bony shield generally.^ I have now only 

 to condense all the observations related above, in a few words. 



The ossifications of the corium in Turtles take place only in the dorsal and 

 ventral walls of the body. Their development is greatest in land Turtles,^ and 

 least in the Trionychidae and Sphargidida?; in which latter, though they are relatively 

 more extensive than in the Trionychidaj, they yet nowhere reach the true skeleton. 

 The deposition of lime in these ossifications is mostly so extensive, that they are 

 just as hard as true bone, and in proportion to this deposition of lime, their 

 structure ai^proaches also more and more that of true bone, the holes of the 

 elastic membrane appearing then as haversian canals, and around them the fine bone- 

 holes, but it shows still everywhere its character as dermal bone by the irregularity 

 of its structure. In order to ascertain what is true skeleton bone, and what dei-mal 

 bone, I have availed myself not only of the difierence in their structure, but resorted 

 also to the investigation of the cartilaginous skeleton in the embryo, or in the young 

 soon after hatching. Such young Turtles furnish, indeed, the most beautiful micro- 

 scopical objects for the study of cartilage and its ossifications. Now wherever we 

 find regular cartilage in the young, we take it for granted that such parts are to 

 be considered as belonging to the true animal skeleton. Thus we have ascertained 



of the so-called skeleton of the Testiuliiiata is formed the true skeleton is afforded by tlie solid frame of 



by the skin. This has been further illustrated by W. Trionyx, in wliieh the growth of the dermal and of 



Peters (Observationes ad Anatoniam Cheloniorum, the true skeleton takes place by an alternate extension 



Berolini, 1838) and by Owen (Observations on the of their respective peripheric parts, as we shall see 



Develoj)mont of the Carapace and Plastron of the fully when considering this family more in detail. 

 Chelonians, Philos. Trans., 1840, and Fossil Eeptilia, ^ See, above, Sect. 4, p. 255-257. 



Palajontographical Society, 1849). The most strik- ^ It is in this sense that the statement on page 



ing evidence of the independence of the dermal and 236, line 22, is to be understood. 



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