MUSEUM OF COMPARATIVE ZOOLOGY. 263 
internal border of the segment (turned away from the reader in the 
figure) are not duplicated on the homologous margin of the upper half 
of the segment.  Articulated with the distal end of this segment are 
two carpi (4, 4’). The supernumerary carpus (4 does not have the 
exact form of the normal carpus (4), but is slenderer, subcylindrical, and 
much more spiny. The normal carpus is followed by a propodus and 
dactylus (5, 6) of the regular form. The supernumerary carpus bears 
at its distal extremity an abortive propodus (5') in the shape of a small 
stump-like segment, bifurcated at the end and armed with a blunt 
spinous tubercle (y^) on its inner margin. This tubercle is homologous 
with the tubercle y at the proximal end of the external border of the 
normal propodus. Curiously, the supernumerary carpus is set upon the 
meros in a position almost the reverse of that of the normal carpus, so 
that the surface of the accessory carpus and propodus, which is homolo- 
gous with the upper surface of the regular carpus and propodus, looks 
in almost the opposite direction. It is as if tho abnormal carpus were 
rotated upon the meros through nearly 180 to the left. It thus comes 
about that the articular tubercle a’ falls on the same side with its 
homotype, æ, instead of on the opposite side, as one would expect from 
the reversed symmetry of the two carpi. If the two propodal segments 
(5, 5!) were flexed at the same time, they would move in nearly opposite 
directions. This distortion seems to me very singular, and I think 
nothing like it has been observed among the many cases of double legs 
in insects. 
In this specimen we have the nearest approach to complete duplica- 
tion of a limb yet observed among Crustacea. It reminds one of the 
monstrosities among insects, frequently described by entomologists, in 
which the duplication of a leg may involve all tho joints down to the 
trochanter. Whether this monstrosity be congenital, or the result of 
injuries received later in life, I cannot tell. 
Plate II. Fig. 7 (Homarus Americanus, left chela). — In this small 
chela only a rudiment of the index is present, and the dactylus is curled 
underneath it in the form of a semicircle. 
Plate II, Fig. 8 (Callinectes hastatus, left lateral portion of the cara- 
pace). — The lateral horn, instead of being simple, as in normal speci- 
mens, has three spines, one directed forward, outward, and downward, 
one backward, outward, and upward, and one, very small in sizo, back- 
ward, outward, and downward. 
Plate IT. Fig. 9 (Homarus Americanus, right chela). — The whole of 
the index as well as part of the hand is wanting in this sadly mutilated 
