284 bulletin: museum of comparative zoology. 



The other skeletal elements of this manus are normal. The muscula- 

 ture and innervation are identical with the conditions shown in Figures 

 Q, R, and S. 



2. Manus in which the Supernumerary Parts may be more or less closely 



connected with Metacarpal II. 



^ a. One Supernumerary Digit. 



This condition was observed in five cases. From a typical example 

 (Plate 17, Fig. 25) it might be inferred that all these cases were to be 

 interpreted as mere duplications of digit ir. The extra digit (i) pos- 

 sesses three phalanges and is of the same size as ii. Both are borne 

 on the same metacarpal, which is large and has two articular condyles 

 at its distal end. The digits, however, are nut symmetrical with each 

 other, as we should expect if they had resulted from duplication of 

 digit II ; in both, the hoofs and ungual phalanges are concave on the 

 ulnar, convex on the radial side. lu the carpus the trapezium is 

 larger than normal, and articulates above with the scaphoid, and below 

 with the radial portion of the proximal facet of metacarpal ir. This 

 condition is represented by only a single case. In four other specimens 

 the skeletal parts exhibited very interesting conditions which serve to 

 connect this class of abnormalities with the first part of the series we 

 are describing. In Figure 26 (Plate 18) it is seen that the extra 

 digit (i) is much larger than the second (ii), but, as in the preceding 

 case, both are borne on a single large metacarpal. They are not sym- 

 metrical with each other, and on examining carefully the metacarpal, 

 a dark irregular line will be seen, running nearly tlie whole length of 

 the bone and dividing it into two unequal portions. This line of separa- 

 tion, so clearly brought out in the skiagraph, is not, of course, a surface 

 marking but represents a complete bony septum. The two components 

 into which the metacarpal is thus divided, correspond in size with the 

 digits which they respectively bear. 



The structure of the carpals furnishes important evidence as to 

 whether the extra digit is formed by the splitting of ii. If this were 

 the case, the trapezoid should show signs of duplication, while the tra- 

 pezium should remain normal. On the contrary the trapezium is large 

 and fused to the trapezoid. Comparing Figure 20 with Figure 17 

 (Plate 9), the similarity of the skeletal structures is striking, and we 

 can but conclude that the manus shown in Figure 26 differs from that 

 shown in Figure 17 only iu the fusion of its trapezium and traj^czoid, 



