208 MARINE SAURIANS. 
as well as its safety, chiefly by means of artifice 
and concealment. 
The ribs are composed of two parts, one ver- 
tebral and one ventral ; the ventral portions of 
one side, (PI. 18, 3, b,) uniting with those on 
the opposite side by an intermediate transverse 
bone, (a, c,) so that each pair of ribs encircled 
the body with a complete belt, made up of five 
parts. t Cuvier observes that the similarity of 
this structure to that of the ribs of Cameleons 
and two species of Iguana, (Lacerta Marmorata, 
Lin. and Anolius, Cuvier,) seems to show that 
the lungs of the Plesiosaurus Dolichodeirus, 
(as in these -three sub-genera of living Saurians,) 
were very large ; and possibly that the colour 
of its skin also was changeable, by the varied 
intensity of its inspirations.^ Oss. Foss. Vol. V. 
Pt. 2. p. 280. 
* See PI. 16, 17, 18. 
t The ventral portion of each rib, (PI. 17, and PI. 18, 3, 
b,) appears to have been composed of three slender bones fitted 
to one another by oblique grooves, allowing of great expansive 
movement during the inflation of the lungs : the manner in 
which these triple bones v;ere folded over one another, is best 
seen in a single series between a, and b, the upper ends of the 
ventral portions of the ribs (b) have been separated by pressure, 
from the lower ends of the vertebral portions, (d.) 
X We have no means to verify this ingenious conjecture, that 
the Plesiosaurus may have been a kind o{ sub-marine Cameleon, 
