508 EMBRYOLOGY OF THE TURTLE. Part III. 
nest of all known to us, being about one half as thick as that of its congener 
Cinosternum; and yet its shell is slightly thicker than that of Chelydra. 
The annexed table will give a graphic view of the relative thickness of the 
shell and shell membrane of the different genera of Turtles. The line which 
runs between the columns, marked “shell” and “shell membrane,” indicates the 
junction of the two. The length of the straight lines on the left shows the thick- 
ness of the shell membrane magnified to five hundred times its diameter; and the 
straight lines on the right, the thickness of shell under the same amplification. 



GENERA AND SPECIES. SupLtt MemBRane. SHELL. Suett Srrucrure. 
Chelonioide. 
Thalassochelys Caouana ic ob oe oe a Nodular, nodules very friable. 
Trionychide. 
Platypeltis ferox «9. «) © 2 © + = = © se Continuous surface. 
Chelydroide. 
Gypochelys Temminckii Soh Ac joel surface, nodules hard 
Chelydra serpentina and brittle. 

Cinosternoide. 
Ozotheea odorata 

, : i Continuous surface. 
Cinosternum pennsylyanicum 

Emydoide. 
Graptemys geographica . 

Ptychemys concinna . . . . . ——— 

Nodular surface, nodules hard 
and brittle. 

Chrysemys picta 
Nanemys guttata 

Glyptemys insculpta . . . . . a 


imys Meleseris: . 0. 6. wb ere its Nodular, but smooth and hard. 
( Nodular surface, nodules hard 
Cistudo virginea 2 
(and brittle. 

Testudinina. 
Xerobates carolinus .... . Continuous surface. 


The Shell. As we have already indicated the thickness of the egg shell, when 
speaking of that of the shell membrane, we will at once pass on to describe the 
mode of development and the structure of this, the most superficial of the different 
layers surrounding the egg. 
By dissolving the carbonate of lime of the shell with nitric acid?’ the basis of 
1 The eggs had not yet matured their shell. oration, deposits groups of crystals, (PI. 18, fig. 11,) 
2 Nitric acid decomposes the carbonate of lime which exhibit the characteristic, long, tabular, rhom- 
very rapidly till it becomes saturated, and, upon evap- bohedral forms of nitrate of lime (fig. 11a, 11b, a, 6). 
