70 Firtp Co_tumpian Musrtum—Geouoey, Vou. II. 
masses, which originally formed the borders or terminations of the 
adjacent bones. These, in the case of the phalanges, have become 
separated to increase the mobility of the fingers, have increased in 
size and ossified, and have in turn given rise to new cartilaginous 
-extremities, that have separated and become ossified; that is, the 
growth of the additional phalanges and carpals, as well as the two 
additional bones of the fore arm, was peripheral. The additional 
fingers of the ichthyosaur may be also explained in a similar way. 
The margin of the flipper has become hardened by fibro-cartilage, 
which by the movement of the fingers was broken into segments, each 
of which finally took on ossification. A division of the phalanges by 
segmentation would certainly have to take place immediately in order 
to preserve the integrity of the paddle as an organ of propulsion. If 
this explanation be correct, then these additional ossifications in the 
fore arm in the present form, as well as in the carpus, are mo? dis- 
placed elements, but new ones, without homologies. 
To return to the description from this digression, the frs¢ 
eptpodial supernumerary is irregularly five-sided in shape, for articula- 
tion with the carpal, ulna, humerus and second epipodial supernu- 
merary, the longest, inner border, being thinned and non-articular. The 
second epipodial supernumerary 1s smaller than the first, and is some- 
what elongate, articulating distally with the- first supernumerary, 
proximally with the humerus, and with its longest, somewhat convex 
and thinned internal border non-articulate. . 
There are four bones in the first row of the carpus, and the same 
number in the second, the inner one of which partakes so strongly of 
the nature of a phalange that it doubtfully can be called.a carpal. 
The first, or radiale, is a thick massive bone, nearly regularly 
parallelogrammatic in shape, and articulates with four bones. The 
broad proximal end articulates with the radius; the distal surface with 
the first distal carpal; nearly the whole length of the inner side joins 
the medial carpal, leaving a small space at the distal angle for union 
with the second distal carpal; the outer border is rounded and non- 
articulate. The second distal carpal is wanting in this specimen, and 
has been restored. It must, however, have articulated with the 
radiale and» mediale proximally, the first distal carpal externally, 
the third distal carpal internally, and the second and third metacarpals. 
A good figure of this bone of the plesiosaur paddle will be found in 
Leidy, Cretaceous Reptiles, plate iv, ff. 13 and 14. The third distal 
carpal begins to assume something of the nature of a metacarpal, 
though a true carpal; it articulates proximally by two unequal facets 
with the ulnare and medial carpals; externally with the second distal 
