[&2 MARINE SAUPaANS. 



Ribs* 



The ribs are composed of two parts, one vertebral 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.f Cu- 

 vier 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 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. J Oss. Foss. Vol. Y. 

 Pt. 2. p. 280. 



• See PI. 16, 17, 18. 



j- The ventral portion of each rih, (Pi. 17, and Pi. 18, 3, b,) appears to 

 have been composed of three slender bones fitted to one another by oblique 

 grooves, allowuig' 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 ven- 

 tral 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 Pie- 

 siosaurus may have been a kind of sub-marine Chameleon, possessing the 

 power of altering the colour of its skin; it must however be admitted tiuit 

 such a power would have been of much advantage to this animal, in defend- 

 ing 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 fre- 

 quency of its ascents to the surface, to inspire air; an operation that must 

 have been attended with constant danger, in a sea, tliickly swarming with 

 Ichthyosauri. Dr. Stark has recently observed that certain fishes, especiall)- 

 minnows, have a tendency to assume the colour of the vessel in which they 

 arc kept. (Proceedings Zool. Soc. Lond. July, 1833.) As in animals of 

 this class there are no lungs, this cliange of colour must arise from other 

 causes than that to which it has been attributed in the Chameleon. 



