262 BULLETIN OF THE 
Near the base the dactylus divides into two branches, a long one (a), 
which appears to be the distal part of the original dactylus bent so as to 
make almost a right angle with its proximal portion, and a shorter one 
which forks at the end (b, c), and presents a row of teeth on both the 
inner and outer borders. This shorter branch has the normal direction 
of the dactylus, and is probably a secondary outgrowth from the primi- 
tive dactylus. This malformation resembles that seen in Plate I. figs. 
7 and 8. "The propodite is lost. 
Plate II. Fig. 5 (Callinectes hastatus, left chela). — The dactylus is 
divided longitudinally, nearly to its base, and furthermore the lower of 
the two branches thus produced forks at a point midway between the 
base and the tip. One of the prongs of the fork (c) inclines toward the 
upper branch of the dactylus (b), the other prong (a) is curved down- 
ward toward the index finger. The dactylus thus becomes tridactyle 
instead of monodactyle. The superior branch (5) is toothed along its 
lower edge, the inferior branch along both its upper and lower edges, 
the teeth of the upper edge being continued along the upper margin of 
the upper prong (c), while the teeth of the lower edge are continued 
along the lower Margin of the lower prong (a). All the branches are 
much shorter than the index finger. The teeth on a do not strike 
against those on the index when the claw is shut. Even the coloration 
of a, b, and c is like that of the normal fingers. 
This monstrosity is like that described and figured by Lucas (Carcinus 
menas, in No. 7 of the Bibliography). I differ from Lucas in the inter- 
pretation of the finger-like parts of the tridactyle segment. He con- 
siders b to be the normal dactylus, and a and c to be supernumerary 
fingers, a being the analogue of the dactylus (5), and c the analogue of 
the index. From the analogy of this deformity with those represented 
on Plate I. figs. 1—5, I conceive « to represent the original dactylus, and 
b and 6 to be the supernumerary parts, representing the dactylus (a) 
and the index respectively. 
Plate IT. Fig. 6 (Homarus Americanus, right cheliped). — The first 
segment (coxa) is wanting. The second and third segments, instead of 
having their normal flattened form, are subcylindrical. The third seg- 
ment (meros) further shows a tendency to divide, a deep groove running 
across the distal end. The upper half of this segment repeats antitropi- 
cally, orin a reverse mamner, the lower half: thus the spine sp on the 
anterior border is symmetrically repeated in sp’, and the articulating 
process z has its homotype in 7. The symmetry of the segment is not 
complete, however, inasmuch as the two or three short spines on the 
