19*5 •] Fauna of the Chilka Lake : Crustacea Decapoda. 295 



male. In the determination of the sex it is essential that the second pleopods should 

 be examined to ascertain whether the appendix masculina is present or absent. 



There is one other point of more than systematic interest in the specimens of 

 Athanas found in the Chilka Lake, and this concerns the development of the second 

 pair of legs. The carpus of these limbs in Athanas is composed of five sub-segments, 

 whereas in the allied genus Arete there are only four. In twenty- three specimens of 

 Athanas polymorphus the carpus is on both sides composed of five sub- segments and 

 has a similar development in the single limb of the pair which alone persists in 

 two additional specimens. The two remaining examples are, however, abnormal. 

 In one of them, a male belonging to Form I, the carpus on one side is five-, and on 

 the other four- segmented, while in the other specimen, which is a female, both the 

 carpi are composed of only four segments (text-fig. 32e). The last specimen, if it 

 had been taken alone, would almost certainly have been described as a new species 

 of Arete, bearing a close resemblance to Athanas. 



It is, however, through the nitescens group of Athanas that Coutière would 

 derive Arete and not through the dimorphus group to which the Chilka species 

 belongs. 



Athanas polymorphus, sp. nov. 



The rostrum is without teeth and reaches almost to, or a little beyond the end 

 of the second segment of the antennular peduncle. In two large males (those belong- 

 ing to Form III) it has evidently suffered injury and is abruptly curtailed at the 

 apex, reaching only to the middle or end of the basal antennular segment. The 

 dorsal carina of the rostrum extends backwards and is visible in the anterior sixth of 

 the carapace. The supra-corneal spine is entirely absent ; the extra-corneal is well 

 developed, reaching to about half the length of the eye. The infra-orbital angle 

 (" dent infra-cornéene " of Coutière) is small, but acute, though less spinous in char- 

 acter than the extra-corneal. There is also a sharp tooth opposite the insertion of 

 the antennae, absent in A. dimorbhus, which must, I think, be the homologue of the 

 pterygostomian spine (text-figs. 32a, b). 



The eyes are small, but well pigmented. The antennular peduncle reaches to 

 the apex of the rostrum. The basal segment is little longer than the second and on 

 its infero-internal margin bears, as is usual, a well-marked longitudinal crest, which 

 terminates anteriorly in a tooth reaching almost to the distal end of the segment. 

 The lateral process (stylocerite) is composed of a long spine which extends only a 

 little beyond the end of the segment. The second segment is about one and three 

 quarter times as long as the third. The inner antennular ramus is longer than the 

 outer : the latter is distally bifid and the thicker of its branches, which is much the 

 shorter, is about as long as the peduncle. The fused part is composed of from 8 to 

 10 segments and the free portion of the outer and thicker of the two branches is from 

 one half to three quarters its length. 



The carpocerite reaches almost to the end of antennular peduncle. The anten- 



