640 EXPLANATION OF THE PLATES; 
Fig. 10. Head of Pl. 16, fig. 1; 4 diam. 
Fig. 11. Left anterior foot, 25 diam.; period of laying un- 
known. 
Fig. 12. Left fore foot, from above, 10 diam.; period of 
laying unknown, opened Aug. 25, 1852. 
Fig. 13. Brain of an adult Chelydra serpentina, from above ; 
fiz. 13a, the same as fig. 13, seen in profile from the left 
side, slightly magnified ; c, cerebellum ; /, hemispheres; i, 
pituitary body; x, nostrils; sm, Schneiderian membrane ; 
*, cut edge of the cartilaginous box surrounding the nasal 
cavity; on, olfactory nerves; ob, olfactory bulb; pg, pin- 
eal gland; cq, corpora quadrigemina; pe, plexus cho- 
roideus; no, optic nerve. A comparison of these two 
figures with those of Plate 23 will readily show the re- 
markable changes which the brain undergoes, even after 
its parts are well defined. A further comparison of these 
figures with those of Plates 12, 18a, and 24, will exhibit 
the whole range of the transformations of that organ, 
from its first appearance to its complete growth. 


PLATE XXVI. and XXVII. 
[Painted from nature, by J. Burkhardt. ] 
These two Plates exhibit the range of variations of the 
Pl. 
— 
1a 
colors in one and the same species, and the succes- 
sive changes in their distribution, as well as in the 
tints. It is the Emys rubriventris Auct., rugosa Shaw. 
26, fig. 1 to 4, represents a young Ptychemys rugosa in 
four different views, just hatched: fig. 5 to 7, three dif- 
ferent specimens, three or four years old, in profile, and 
from above, two thirds the natural size ; fig. 8, specimen 
five or six years old, reduced nearly one half, from be- 
low; fig. 9, specimen about 7 years old, reduced one 
third; fig. 10 and 11, half grown specimens, about ten 
or twelve years old, reduced one half. 
27, fig. 1 to 3. Full-grown specimens, reduced one half ; 
fig. 4, the eye, natural size; fig. 5, front view of the 
head, reduced one half. 

