590 
Mr. De la Beche and Mr. Gonybeare on 
whereas in the Plesiosaurus the comparative shortness of the in- 
dividual joints is much more than compensated by their exceedingly 
greater number, and a much greater flexibility is thus imparted to the 
whole paddle. 
Here then we may trace a series of links between the extremities 
of land and sea animals. The sea turtle will form the first term of 
these, in which the arrangement and form of the carpal, meta- 
carpal, and phalangic bones, will be found to differ from other 
quadrupeds, and approach to the Plesiosaurus in many points, while 
yet their number and general appearance more nearly resemble the 
usual quadrupedal type. In the Plesiosaurus these peculiarities have 
added to them that of a much greater number of joints, and an 
external series of rounded bones. In the Ichthyosaurus the usual 
phalangic form is completely abandoned, and all the bones become 
irregular polygons or trapezoids, thus admitting a still greater 
number of separate joints into the same space, and proportionally 
encreasing the flexibility. In the fins of regular fishes, flexibility is 
also the object apparently aimed at, and produced by the number of 
joints and their cartilaginous substance, but their form more nearly 
resembles that of ordinary phalanges. We have ventured to add a 
figure of the paddle of the Plesiosaurus, as conjecturally restored from 
the two fragments, and the analogies alluded to, fig. 5, plate 42. 
The general arrangement of the whole extremity is also im- 
portant, as exhibiting a similar gradation of arrangement ; for with 
the flat clavicular bones of the Ichthyosaurus, and with the inter- 
mediate species of paddle above described, we find the distinct form 
of humerus, radius , and ulna, belonging to quadrupeds generally. 
Of the posterior extremities, we have seen some imperfect bones 
of the pelvis and femora ; but we postpone any attempt to describe 
this part of the animal, till future opportunities shall have provided 
us with more illustrative specimens. 
