AN ABERRANT LIMB IN A CRAY-FISH. 8 I 



normal claw as well as a pronged structure that simulates a claw 

 even in details and was probably movable after the manner of a 

 dactyl. This pronged structure is remarkable for its symmetry. 



Comparing this with other described cases we find in the first 

 place that it is unusual in being upon a walking leg. Of the 

 thirty-one cases of abnormal appendages quoted by Bateson, 1 two 

 are of antennae, four are of non-chelate legs, and all the rest of 

 chelae except one, which is of a chelate walking leg. 



Of the eleven additional cases given by Herrick 2 only two are 

 of walking legs. However, this relative infrequency of described 

 abnormalities in walking legs may be due, in part, to the greater 

 ease with which other cases are collected or noted. 



In the second place, it is unusual in being a monstrosity of the 

 propodite. Bateson found the greater number of cases of repe- 

 tition of parts, in the Crustacea, are repetitions of extra dactyls 

 upon a normal dactylopodite (some fifty cases), and that next in 

 frequency are the cases of extra index upon a normal index (some 

 fifteen cases). 



In the third place it seems to fall into none of the four cate- 

 gories established by Bateson, but rather to be like the excep- 

 tions, of which he found only two. 



It has, however, resemblance to the case 815 of Bateson and 

 more to the case shown in Fig. 195 of Herrick and still more to 

 the case shown in Fig. 2 of Faxon, 3 which was described as 

 follows : " 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 appears to have been 

 budded off from an amputated surface of the propodite. It con- 

 sists of two fingers which have the form of the normal dactyl us 

 and index, but neither is articulated with the other at the base. 

 The two fingers together seem to be morphologically equivalent 

 to a single segment, and represent a two-branched supernumerary 

 dactyl us." 



Though the pronged structure we have described is markedly 

 like a claw in its symmetry yet any tentative attempt to interpret 



1 Bateson, " Materials for the Study of Variation," 1894. 



2 Herrick, "The American Lobster," Bull. U. S. F. C., 1895. 



'Faxon, "On Some Crustacean Deformities," Bull. M. C. Z., VIII., 1880. 



