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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
which it is a part. According to my interpretation, such a case as that shown in fig. 
193 has gone through phases similar to those shown in figs. 189-191. 
In figs. 192 and 197 the conditions are somewhat different, since the superadded 
dactyl is single. I think there can be no doubt that a progressive division of the 
propodus takes place in such cases. In fig. 192 the plane of fission in the propodus is 
marked by spines very much as in fig. 190. There may be a line of median, unpaired 
spines at the bottom of the groove, and bilaterally symmetrical spines upon its sides. 
It seems probable that the conditions like those seen in fig. 197 could be derived from 
such as are met iu fig. 192 by the intervention of a single molt. 
Faxon describes a very interesting case {66, plate 2, fig. 6) in which there appears 
to be a duplication of the right cheliped down to the meros. The latter is partially 
divided by a deep groove running across its distal end. It seems to me very 
probable that we have here an illustration of the same process which is seen in 
figs. 192, 197, only carried a step or two farther. In the former case the fourth joint 
of the limb is undergoing a process of fission begun nearer the outer extremity, while 
in the latter the sixth segment is involved. The supernumerary carpus iu the case 
figured by Faxon bears a stump-like segment, which looks like an abortive propodus, 
corresponding probably to the abortive segment borne on the extremity of the super- 
numerary branch of the propodus in fig. 197. In the latter case it is an undoubted 
dactyl, and is smaller and more rudimentary than iu fig. 192, where the fission of the 
propodus has not gone so far. It is thus probable that with the extension of this 
process, emphasized at each molt, the terminal segments may in some cases, as in 
those before us, atrophy and disappear, until we have, as in the example cited by 
Faxon, only an abortive propodus left. Bateson regards this superadded member 
as double, formed of two compounded parts. This may be so, but the same kind of 
reasoning would lead us to regard such an incipient member as that seen in fig. 197 
as double, consisting potentially of two dactyls and two propodi. The only apparent- 
reason for doing so lies iu the supposition that such a superadded part arose as a 
tubercle or budding growth on some part of the claw, probably in this case on the 
dactyl, and was potentially a double member from the start, or at least callable of 
doubling by a process of fission, as we see actually going on in fig. 190. Whatever 
changes may have taken place precedent to the condition seen in fig. 197, there is no 
evidence of fission in the extra dactyl unless the two spines ( 8 , S 1 ) be taken as such. 
There seems to be a considerable gap between the condition seen in fig. 193, where 
three dactyls are present, one of which is free, and t-liat shown in fig. 192, where there 
is a siugle process. The latter is bent downward and toward the primary dactyl. Its 
inner border has a spine {S) like that borne on the normal dactyl, showing this part 
to belong to the left side. It bears also another spine near its articulation with the 
propodus ( 8 1 ) , which might indicate that this toothless appendage was really a 
double member. (See fig. 197, 8, S 1 .) 
Another good example of repetition of the propodus, with division of the bud, is 
shown in figs. 187, 188, plate 46, which are from photographs. In this case the 
bud has grown out obliquely from the under side of the propodus instead of from the 
margin, as in fig. 190. The continuity of the outer margin is interrupted by a deep 
groove which divides the bud into perfectly similar parts. In this case the teeth on the 
inner margins of the supernumerary digits are not opposed. The outer or lowermost, 
which is usually symmetrical with the normal part, makes here an angle of about 42° 
with the normal digit, and the two supernumerary digits make an angle of 12° with 
