AN ABERRANT LIMB IX A CRAY-FISH. 77 



movement of 2.5 mm. in the plane of the real forceps, one half 

 of this being towards the forceps and one half away from it. 

 There is also mobility at right angles to the above plane, in a 

 general antero-posterior direction. Anteriorly this is 1.5 mm. 

 and posteriorly about .7 mm., a total swing of 2.2 mm. On 

 moving the abnormal structure and the normal dactyl as far as 

 possible toward one another they came just into contact. The 

 angle of divergence between the index and the most remote part 

 of the abnormal structure is greater than the extreme opening of 

 a normal claw. 



The form of the pronged structure is remarkable for its sym- 

 metry : the two prongs (Fig. 2) differ but slightly in shape and 

 in size. But the one standing nearer to the claw is slightly 



FIG. 3. Camera sketch of an anterior view of a terminal part of the third left 

 leg of a normal C. Bctrtoni. 



thicker and less sharply pointed and it also lacks the clear, per- 

 forated horn-like tip present upon the other prongs as upon 

 normal claws. That this tip was lost by wear or by accident 

 seems evident from the rough surface ending the prong and from 

 the fact that staining liquids easily enter at this point. 



Since the anterior and posterior faces of the prongs are not 

 alike one could not put either prong into a space of the shape of 

 the other prong ; the two prongs are symmetrical about a plane 

 between them ; they are mirror images of one another, except for 

 the above noted difference in form. This symmetry extends to 

 such details as the distribution of clusters of bristles or hairs and 

 to the arrangement and number of the serrations along the op- 

 posed faces of the prongs. 



This latter detail deserves special description. These serrations 

 are like those along the opposed faces of the dactyl and the index 

 and they add greatly to the impression that the pronged structure 



