=I 
Cuap. II. ALBUMEN AND EGG SHELL. 50 
ular bodies, which, in the shell-membrane, become the components of the fibres 
Each granule has the appearance of being composed of three or four superposed 
concentric coatings, reminding one of the structure of starch granules. Now, what 
these apparent layers are has not been ascertained; but it does not seem possi- 
ble that they should be the result of refraction, for that, as far as we know, would 
not produce more than one dark band, or a central dark spot, whilst bere we 
perceive at least two bands within the outline and a central cloudiness, so that 
we feel justified in saymg that in all probability they are concentric concretions. 
But this needs further investigation. 
The thickness of the shell membrane varies greatly: in some species it attains 
to a very great crassitude ; while in others it amounts to hardly one third the 
extent of the first. Thus, in Thalassochelys Caouana (Pl. 7, fig. 30) it is more 
than one half thicker than in Chelydra serpentina, (Pl. 7, fig. 24-26; Pl. 9a, fig. 
43, ¢, d,) but with a quite thm and friable shell; whilst in Gypochelys Temminckii 
(Pl. 7, fig. 27) the shell membrane equals that of T. Caouana, but the shell is 
as thick as that of C. serpentina, and rather more dense than the latter, the cal- 
careous nodules being more closely packed together. 
Chelydra serpentina (Pl. 7, fig. 24-26) has the next thickest membrane, (PI. 
9a, fig. 43, ¢ d,) beimg almost two thirds as thick as in Thalassochelys, and a 
shell (a) equal to that of Gypochelys. Emys Meleagris (Pl. 7a, fig. 26, and 27) 
and Xerobates carolinus (Pl. 7, fig. 28, and 29) have a membrane of about the 
same thickness with C. serpentina; but the shell of E. Meleagris equals that of 
C. serpentina, while that of Xerobates is more than one half thicker than in 
these two species, and far more dense and _ brittle. 
7 
Glyptemys insculpta (Pl. 7a, fig. 15-17) and Ptychemys concinna (PI. 7a, fig. 
20-23) also have shell membranes as thick as the last; but the shell of Ptyche- 
mys is two thirds as thick as in Chelydra. Graptemys geographica, (Pl. 7 
28-30,) Nanemys guttata, (Pl 7a, fig. 7-10,) and Cistudo virginea, (Pl. 7, fig. 
10-14,) have shell membranes hardly as thick as in Chelydra, and the shell half 
that of the latter. 
Cinosternum pennsylvanicum (PI. 7, fig. 1-6) has a shell membrane two thirds 
a, fig. 
as thick as in Chelydra; its shell, however, nearly equals, in this respect, that of 
Xerobates carolinus, but is more dense and brittle than the latter. 
Chrysemys picta (Pl. 7a, fig. 1-5) has a shell membrane and a shell equal to 
one another, as regards thickness; and both together only equal the shell mem- 
brane of Chelydra. The shell membrane of the ege of Platypeltis ferox (Pl. 7, 
fig. 2) is about equal to that of Chrysemys; but its shell is much thicker, about 
one sixth thicker than that of Chelydra. 
The shell membrane of our Ozotheca odorata (Pl. 7, fig. 7-9) is much the thin- 
