146 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



which it is a part. According to my interpretation, such a case as that shown in tig. 

 193 has gone through phases similar to those shown in tigs. 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 in 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 in 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 in 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 wovdd 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 in 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 capable 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, 8 ] ) 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 that shown in fig. 192, where there 

 is a single process. The latter is bent downward and toward the primary dactyl. Its 

 inner border has a spine (8) 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 (S 1 ), which might indicate that this toothless appendage was really a 

 double member. (See fig. 197, 8, 8 l .) 



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 



