MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 389 
the magnum, the third metacarpal, and phalanges of the third digit. The fourth margi- 
nal ossicle, c” (the future trapezium) would have attached to one side of its distal end 
a chain of ossicles (c, m’, &e.), the future trapezoides, the second metacarpal, and the 
i Fig. 3. Fig. 4. 
Fig. 1. 
A 
AL 
Fig. 1. Limb-bones of Ichthyosaurus. Fig. 3. Ideal fore limb of Amphibian. 
Fig. 2. Limb-bones of Plesiosaurus. Fig. 4. Ideal typical limb developing from a fin. 
H. Humerus. R. Radius. U. Ulna. r. Scaphoides. i. Lunare. w. Cuneiforme. c. Os centrale (represented as 
divided). c'. Trapezium. c^. Trapezoides. c". Os magnum. ce‘. One part of unciforme. c’. Other part of unci- 
forme. m'-*, Metacarpal bones. p^. A sixth chain of ossicles, of which the pisiforme is supposed to be the last 
phalanges of the second digit,—the fifth marginal ossicle (the future first metacarpal) 
having attached to it other marginal ossicles, the future phalanges of the pollex in the ` 
manus, or of the hallux in the pes. 
This view is applied to the limb-bones as they exist in Ichthyosaurus, Plesiosaurus, and 
to an ideal amphibian, as the woodcuts sufficiently explain. 
The hypothesis is very ingenious and tempting; still, much as one is naturally dis- 
posed to accept it, there are yet certain difficulties which it will not do to ignore. 
In the first place, the exact nature of some of the ossicles of Ichthyosaurus, together with 
the division of the os centrale, seems yet open to dispute. And again, though the pisiforme 
is accounted for by reckoning it as the rudiment of a sixth digit, yet there is no way of 
accounting for the presence of the corresponding ossicle on the radial side; it cannot 
be similarly explained, because it is on the outer side, and would be external to the 
series of original marginal ossicles; nevertheless this radial ossicle is very often present 
even in Primates*, and in the Mole it becomes enormous. Moreover, as I have been re- 
minded by Professor Huxley, some specimens of Ichthyosaurus not only have extra ossi- 
cles along the ulnar side of the limb, but along the radial side also, and extending far 
up its radial border. A fine specimen showing this has been pointed out to me at the 
British Museum by Mr. Davis. . 
* Phil. Trans. 1867, p. 319. 
