162
MARINE SAURIANS.
Ribs.[1]
The ribs are composed of two parts, one vertebral and one ventral; the ventral portions of one side, (Pl. 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.[2] Cuvier observes that the similarity of this structure to that of the ribs of Chameleons 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 subgenera of living Saurians,) were very large; and possibly that the colour of its skin also was changeable, by the varied intensity of its inspirations.[3] Oss. Foss. Vol. V. Pt. 2. p. 280.
- ↑ See Pl. 16, 17, 18.
- ↑ The ventral portion of each rib, (Pl. 17, and Pl. 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 were 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.)
- ↑ We have no means to verify this ingenious conjecture, that the Plesiosaurus may have been a kind of sub-marine Chameleon, possessing the power of altering the colour of its skin; it must however be admitted that such a power would have been of much advantage to this animal, in defending it by concealment from its most formidable enemy the Ichthyosaurus, with which, its diminutive head and long slender neck, must have rendered it a very unequal combatant, and from whose attacks its slow locomotive powers must have made escape by flight impossible; the enlarged condition of the lungs, would also have been of great advantage in diminishing the frequency of its ascents to the surface, to inspire air; an operation that must have been attended with constant danger, in a sea, thickly swarming with Ichthyosauri. Dr. Stark has recently observed that certain fishes, especially minnows, have a tendency to assume the colour of the vessel in which they are kept. (Proceedings Zool. Soc. Lond. July, 1833.) As in animals of this class there are no lungs, this change of colour must arise from other causes than that to which it has been attributed in the Chameleon.
