MUTATIONS IN CRUSTACEA OF THE FAMILY ATYID;E. 789 



not be regarded as of generic value, it may be pointed out 

 that, as a rule, though not in every case, they are coincident 

 with other features which help to characterise, although 

 they do not define, the generic groups; and further, there is 

 no criterion by which the generic value of a character may 

 be estimated, except that of its constancy throughout a 

 group of species. 



Bouvier's discovery may be shortly expressed by saying 

 that certain species were found to be dimorphic and to 

 oscillate, as it were, in a state of unstable equilibrium between 

 one generic group and the next. Thus, Miss Rathbun ('01) 



Text-fig. 3. 



Atya bisnlcata. A!, A", First and second chelipeds of the 

 Atya-form. B\ B", First and second chelij^eds of the 

 Ortmannia-fonn (Ortmannia Henshawi). x 7. From 

 specimens in the 



Challenger " collection from Honohilu. 



had described a new species, Ortmannia Henshawi (Text- 

 fig. 3, B', B"), found in association with Atya bi sulcata 

 {A', A"), on the island of Hawaii; Bouvier pointed out that 

 this association was not accidental, but constant, that the two 

 forms were indistinguishable, except by the characters of the 

 chelipeds, and that they should be regarded as constituting 

 a single dimorphic species. He found a similar phenomenon 

 in the case of Atya serrata, described by Spence Bate 

 from specimens obtained by the "Challenger" Expedition at 

 the Cape Verde Islands, and since found in many localities 

 on the islands of the Indian and Pacific Oceans, To the 

 Ortmannia -form of this species Bouvier gave the name 



53§ 



