294 Memoirs of the Indian Museum. [Vol. V, 



believe, most improbable, for the specimens of Form I were not obtained during the 

 breeding season and in the non-breeding season occurred apparently to the exclusion 

 of Forms II and III. It is clear, too, that the instance is not merely one of a 

 seasonal sexual dimorphism such as occurs in Cambarus and Pandalus, though this 

 may in part account for the peculiarities which have been noted. 



On the evidence which I am able to offer, a parallel with the Oxyrhynch crabs 

 also cannot be maintained, for Form I, which approximates most nearly to the 

 female and might therefore be regarded as the representative of Smith's " middle" 

 crabs, is not intermediate in size between Forms II and III, nor do these last forms 

 show the marked difference in dimensions that one would expect if they corres- 

 ponded respectively to the " low" and ff high" forms in the Oxyrhynchs. Simi- 

 larly it is impossible to regard the specimens of Form II as " middle" individuals: 

 the measurements do not tally and the examples were found during the breeding 

 season, at which time Form I was apparently absent. 



The widespread though rare occurrence of a seasonal sexual dimorphism in the 

 Decapoda suggests that this phenomenon will afford a partial explanation of the 

 three forms of male in the Chilka species of Athanas. Form I, even though the 

 appendix masculina is to all appearances fully developed, is probably a non-breeding 

 phase of the sex. It is likely, on the other hand, that Forms II and III are breeding 

 phases and, from the scanty evidence available, it seems reasonable to regard them 

 as definitive dimorphic forms. This theory, the second of those listed on p. 292, ap- 

 pears to me the most plausible of any. 



With further material it will be possible to determine if it is correct, and we may 

 also be able to discover if Form I comprises specimens which have never bred or is 

 a phase of a form that was once sexually active. From the specimens available it 

 seems on the whole most probable that the former explanation is the true one and 

 that males of Forms II and III perish at the close of the breeding season. 1 



The knowledge of the existence of three distinct forms of male in Athanas poly- 

 morphous must, I believe, have a marked effect on our views as to the systematic 

 treatment of the genus, for it is not unlikely that different forms of the same species 

 have been described under separate names. 



I am inclined to think, also, that sufficient care has not been taken in determin- 

 ing the sex of the specimens described. When two forms of a species are found, that 

 with the most highly developed limbs is considered to be the male and the other the 

 female. Even Dr. de Man in his account of Athanas sibogae describes a specimen 

 which " is considered to be the female of this species, with some doubt, because it 

 carries no eggs " ; it is not improbable that this example is really a second form of the 



1 Eventually it may perhaps be possible to find some analogy between the phenomena found in 

 Athanas and those discovered by Sewell in Copepoda. Sewell, in tracing the development of certain 

 species of this group by the application of " Brooks' Law ", has found that in the male there may be 

 two definitive dimorphic forms, both mature. Individuals of stage IV of Sewell's terminology may 

 develop directly into the " low " form or, with the intervention of an additional immature stage, may 

 reach the "high " form. See Sewell, Rec. Ind Mus., VII, p. 316 et seq. (1912). 



