EXCEPTION TO BATESON's RULE OF SECONDARY SYMMETRY. 79 



appendage were in one plane and all three dactyls moved to meet 

 their opposing indices in this plane. 



Another point of considerable interest is the color relation of 

 the extra portions. In the double extra protopodite and the 

 two extra dactyls the pigmentation was completely reversed, 

 giving as a result a light colored upper and a densely pigmented 

 lower surface. A peculiar conical protuberance present on the 

 light upper surface of the double protopodite was practically 

 free from pigment. 



THEORETICAL CONSIDERATIONS. 



The present example, like most abnormal crustacean appen- 

 dages, falls into the category established by Bateson ('94), 

 "in which the extra limb or extra parts of a limb are themselves 

 morphologically double," but unlike the others it does not 

 conform to the rules of secondary symmetry formulated by the 

 same author. According to these rules the normal appendage 

 and the extra parts lie in the same plane and "the nearer of the 

 two extra appendages is in structure and position formed as the 

 image of the normal appendage in a plane mirror placed between 

 the normal appendage and the nearer one, at right angles to the 

 plane of the three axes; and the remoter appendage is the image 

 of the nearer in a plane mirror similarly placed between the two 

 extra appendages" (p. 479). 



In the case under discussion the pair of extra chelae are mirror 

 images of each other but the appendage nearer the primary claw 

 is not, as the rules provide, a mirror image of the latter. More- 

 over, it does not appear possible to explain this exception to the 

 rules of secondary symmetry by a reference to torsion. Emmel 

 ('07) and Cole ('10) were unable to explain apparent exceptions 

 to Bateson's rules by making allowances for changes in position 

 due to possible torsion. It has previously been pointed out that 

 the coloration of the abnormal double structure was completely 

 reversed, but even if the extra segments were rotated so that 

 the dark surface became uppermost the relations of the claws 

 as regards secondary symmetry would remain unchanged since 

 the double extra claw is bilaterally symmetrical. 



The specimen here described while exhibiting many points of 



