jo Field Columbian Museum — Geology, Vol. 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 not dis- 

 placed elements, but new ones, without homologies. 



To return to the description from this digression, the first 

 epipodial 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 is 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 radiate, 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 



