ICHTHYOSAURUS. 141 



braa constructed after the manner of fishes, had they been 

 furnished with legs instead of paddles, could not have moved 

 on land without injury to their backs.* 



Ribs. 



The ribs were slender, and most of them bifurcated at 

 the top : they were also continuous along the whole verte- 

 bral column, from the head to the pelvis, (see Plates 7, 8, 

 9 ;) and in this respect agree with the structure of modern 

 Lizards. A considerable number of them were united in 

 front across the chest : their mode of articulation may be 

 seen in PI. 14. The ribs of the right side were united to 

 those of the left, by intermediate bones, analogous to the 

 cartilaginous intermediate and sternal portions of the ribs 

 in Crocodiles; and to the bones which, in the Plesiosaurus, 

 form what Mr. Conybeare has called the sterno-costal arcs. 

 (See PI. 17.) This structure was probably subservient to 

 the purpose of introducing to their bodies an unusual quan- 

 tity of air ; the animal by this means being enabled to re- 

 main long beneath the water, without rising to the surface 

 for the purpose of breathing.f 



* Sir E. Home has farther remarked a peculiarity of the spinal canal, 

 which exists in no other animals ; the annular part (PI. 12, D a. and E a.) 

 being neither consolidated with the body of the vertebra, as in quadrupeds ; 

 nor connected by a suture, as in Crocodiles ; but remaining always distinct, 

 and articulating by a peculiar joint, resembling a compressed oval ball and 

 socket joint, (D g. and E g.) And Mr. Conybeare adds, that this mode of 

 articulation co-operates with the cup-shaped form of the intervertebral joints, 

 in giving flexibility to the vertebral column, and assisting its virbratory 

 motions; for, had these parts been consolidated, as in quadrupeds, their 

 articulating processes must have locked the whole column together, so as to 

 render such a motion of its parts impossible; but by means of this joint every 

 part yields to that motion. The tubercle by which the transverse apophysis 

 of the head of the rib articulates with the vertebra, is seen at d. 



+ The sterno-costal ribs probably formed part of a condensing apparatus, 

 which gave these animals the power of compressing the air within its lungs. 



