270 ON A CASE OP HOM020SIS IN THE GENUS ASELLUS. [Mai\ 20, 



The palp is misshapen and rudimentary. It consists merely of 

 an irregular process with slight traces of segmentation. 



The molar process is rather less than the normal in length. Its 

 triturating end is ill-formed, and does not bear the elaborate 

 structures found in the same place in the normal mandible, the 

 apex being simply covered with a thick plate of chitin. 



Fig. 2. 



Apex of extra mandible of Asel/us aquaticus, to show details. 

 jj', palp of abnormal mandible. 



This mandible was in life carried flat to the body, as shown in the 

 drawing, but 1 have not succeeded in determining which morpho- 

 logical surface was upwards. It is not evident which this should 

 be, but presumably it is the face, which in the normal mandible is 

 next the mouth. I regret that I could find no feature sufficiently 

 differentiating the two surfaces ; but from the fact that, as shown 

 in the figure, the diminishing series of teeth are on the exposed 

 surface, it seems likely that the presumption referred to is correct. 

 In a normal mandible this series is much less clearly defined. 



The animal was approaching a moult, and the newly-formed 

 skin, which could be seen through, showed that at the next ecdysis 

 the appendage would not be noticeably changed. 



In the details of the toothing and in the number of the plumose 

 setae (11 instead of 13), the animal departs slightly from Sars's 

 type; but on examining other specimens I find that these characters 

 vary a good deal, as might be expected. 



Elsewhere 1 1 have discussed the significance of cases of this sort, 



1 Especially ' Materials for the Study of Variation,' Chap, v., where an account 

 of previously recorded cases is given. To these should be added a case of an 

 Astacus having a somewhat aniennuliform structure with two jointed filaments 

 replacing one of the eyes : Hofer, B., Verb. d. deutsch. zool. Ges. it. 1894, p. 82, 



