MUSEUM OF COMPARATIVE ZOOLOGY. 261 



plane nearly at right angles with the plane of motion of the normal dac- 

 tj'lus (a). Here, then, in addition to the duplication of the dactyhis 

 seen in Fig. 12 (a' indicating homologous parts in the two figures), one 

 sees an imperfect attempt to duplicate the propodite in the process x. 



Plate I. Fig. 17 (left chela). — This monstrous claw is similar to the 

 one described and figured by Lucas {Ilomarus vulgaris, in No. 7 of the 

 Bibliography). The dactylus (a) does not close upon the index. Fi-om 

 the base of the index there arises from the upper side a very large un- 

 joiuted appendage, which shows a strong tendency to divide into two 

 branches (6, c), each furnished with a row of teeth. The teeth of the 

 branch h point toward the teeth of the index, while those of the branch 

 c are directed toward the row of teeth on the dactylus when the latter 

 is opened. The tendency seems to be to duplicate the dactylus in h, the 

 index in c. As there is no articulation at the base of the monstrous ap- 

 pendage, the teeth on the branch h are useless, and as the branch c is 

 not in the plane of motion of the dactylus its teeth are likewise func- 

 tioidess. Thus, although these two extra lines of teeth are developed, 

 there are no two in the claw which can be applied to one another. 



Plate II. Fig. 1 {Homarus Americanus, dactylus of right chela).* — 

 Beyond the middle, this dactylus is bent downward at nearly a )-ight 

 angle. From the upper side are developed two processes (6, h'), which 

 are forked at their ends and furnished with two rows of teeth within. 

 The propodite is lost. Kesembles the dactylus of the claw figured on 

 Plate I. fig. 15, but differs in the fission of the processes b and b'. 



Plate II. Fig. 2 {Homarus Americanus, one of the small chelipeds). — 

 This leg is provided with two chelae. One of them has the ordinary 

 form and structure, but is bent at a strong angle with the long axis of 

 the leg. The second claw appears to have budded off from an ampu- 

 tated surface of the propodite. It consists of two fingers, which have 

 the form of the normal dactylus and index, but neither is articulated 

 with the other at the base. The two fingers together seem to be mor- 

 phologically equivalent to a single segment, and represent a two- 

 branched supernumerary dactylus. 



Plate II. Fig. 3 {Homarus Americamis, left chela). — In this small 

 chela the index is cui-ved sharply upward and deeply channelled on its 

 lower face. Unlike all those previously noticed in this paper, this is a 

 simple malformation through distortion, without any development of 

 accessory parts. 



Plate II. Fig. 4 {Homarus Americanus, dactylus of right chela). — 



* All the figures on Plate II. are of natural size. 



