PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 287 
This abnormality may be accounted for in two ways: Either (1) digit 
I* represents the developed pollex, and 1° is formed by the duplication 
of digit 1, or (2) digits 1* and 1° are duplications of the pollex, and the 
metacarpal of 1’ is secondarily fused to that of digit mu. The first 
hypothesis is supported by the similarity in structure of digits 1° 
and u, their symmetry with reference to each other, and the differences 
existing between 1* and 1°, The second view, however, is supported 
(1) by the fact that the extra digits are enclosed in the same hoof, and 
therefore probably developed together, (2) by the fact that the trape- 
zium articulates with the compound metacarpal, and (3) by the structure 
of the muscles and nerves. 
To another type belong two cases in which digit 1* is completely 
developed and articulates with the carpus (Plate 20, Fig. 28). Digits 
i and m are borne on a single large metacarpal, but 1° is much the 
larger. The phalanges of 1 are of normal size and unsymmetrical 
with those of 1°. The ungual phalanges of both 1° and 1° are enclosed 
in separate hoofs, and are symmetrical with each other, although differ- 
ing somewhat in size. The trapezium is large, and articulates with 
metacarpal 17 and with a portion of the compound metacarpal. The 
musculature and innervation of this manus are similar to those of the 
foregoing case. 
Our view that these abnormalities are due to duplication of the 
pollex and the subsequent fusion of the metacarpal of 1° to that of 1, 
is favored by the structure of a manus figured by Otto (’41, Tab. 
26, Fig. 12). In this case there are two extra digits of three pha- 
langes; 1° is borne on a distinct metacarpal, which articulates with 
the trapezium, and 1° on a metacarpal which is almost completely fused 
to metacarpal 1. Digit 1 is of normal size. The phalanges of 17 and 
1° form a single series of three bones, each of which is incompletely 
divided into two; the ungual phalanx evidently bore a single hoof. 
The trapezium articulates with metacarpal 1* and with two-thirds of 
the proximal surface of the compound metacarpal. The trapezoid is 
smaller and articulates with the remaining third of the proximal facet 
of the large metacarpal bone. In this manus, therefore, the digits 
1* and 1° evidently developed together, and the fusion of metacarpal 
1° to that of 11 was of subsequent occurrence. This being the fact, it 
is very probable that the foregoing cases which we have examined were 
produced in a similar manner. 
Having now briefly described the types of digital variation in the manus 
of the pig, we shall next attempt to determine their significance. 
