578 
Mr. De la Beche and Mr. Conybeare on 
The whole of this structure, the form of the sternal arch, and the 
broad surfaces of the clavicles, is such, as to impart great strength 
to the chest, enabling the animal to breast the most disturbed 
waters, and affording an extensive surface for the attachment of 
powerful muscles to assist in moving the anterior extremities. 
The glenoidal cavity receives a bone which corresponds at once 
with humerus, radius and ulna, these parts being shortened in 
consequence of their adaptation to support an extremity corre- 
sponding, in its employment at least, rather to a fin than a foot. 
Somewhat similar takes place for the same reasons in the humerus, 
radius, and ulna of the porpoise, which are in like manner 
shortened and anchylosed ; but still in that animal traces of their 
distinct form may be recognised. 
This humero-radius supports the bones of the paddle, in which 
the first four or five rows may be considered as carpal and 
metacarpal, and the remainder as phalangic, although all the bones 
are nearly similar in form, being irregular flattened polygons. 
They are so disposed that the first row consists of only two pieces ; 
the succeeding rows, as far as the fifth, which thus contains six, 
encreasing in arithmetical proportion ; in some specimens eight 
may be counted in the succeeding rows ; but the exterior pieces 
are very small and generally removed ; in length there are about 
thirty-five of these rows, decreasing to a very small size near the 
end : the bones of the exterior series have a more rounded outline 
than the rest ; the longitudinal series of these phalanges proceed in 
right lines from their base in the carpus ; the two first bones of the 
carpal part much resemble that of the porpoise, and the two 
succeeding series present an absolute identity of form and arrange- 
ment with those of the turtle, to which, as we shall hereafter see, 
the paddle of the newly discovered plesiosaurus bears also throughout 
a very close analogy. 
