238 Records of the Indian Museum. [Voiv, XIII, 



Among the Decapoda Macrura characteristic differences be- 

 tween abyssal and shallow-water forms are less evident, but in a 

 number of species belonging to several different families the walk- 

 ing legs tend to become extremely long and slender. This tendency 

 reaches its maximum development in the very peculiar forms 

 belonging to the Nematocarcinidae, a family known onl}'' from great 

 depths of the ocean. 



Only a few species of Macrura inhabit the bed of the Matlah 

 River. The Penaeids do not show any special structural modifica- 

 tions, but in all the Car ids the legs are noticeably longer and very 

 much more slender than is customary. The most remarkable 

 form is unquestionably Leander tenuipes, in which the legs are of 

 the most extreme length and tenuity. I do not know of any shal- 

 low-water Carid ' that is in the least degree comparable with this 

 peculiar species; to find analogous cases we must turn to deep-sea 

 forms and in particular to the genus Nematocarcinus. The modi- 

 fication is not effected in the same way in both cases. In L. ten- 

 uipes it is in the main brought about by the extreme attenuation 

 of the propodus and dactylus ; in Nematocarcinus these two seg- 

 ments are short, the great length of the leg being due to an 

 elongation of the ischium, merus and carpus. 



There are two characteristic features of a deep sea fish and 

 Crustacean fauna that are not met with in the Matlah River, --(i) the 

 eyes are about normal in size and no species, except perhaps P. 

 paradiseus, is even partially blind, (ii) none of the species possess 

 luminous organs. In many deep-sea forms, however, the eyes 

 show but little modification, and it now seems probable that all 

 abyssal animals that possess definite luminous organs or photo- 

 phores (as opposed to glands excreting a luminous fluid) are meso- 

 or bathypelagic in habit and do not live on the bottom. Harpodon 

 nehereus is said to be brilliantly luminous over its whole surface, 

 but this is a statement that I am not able to corroborate by ob- 

 servation. 



Summarising what has already been said, it may be stated 

 that the comparatively small number of animals living on the bed 

 of the Matlah River present modifications similar to those found 

 in a deep-sea fauna. The resemblance is due largely to colour — 

 the similarity in this respect being almost exact — , while it is 

 enhanced in a number of instances by the presence of structural 

 peculiarities rarely met with except in abyssal forms. 



It should be noted that no single member of the Matlah 

 fauna is restricted to the Gangetic delta; some of the species 

 have a wide distribution and several are known to occur in the 

 open sea. It is, moreover, probable that a number are migratory 

 forms, visiting the Bay of Bengal during the flood season; there 

 is, at any rate, evidence that this is the case with the two species 

 of Leander. 



i Except Leander hastatus, AurivJllius, from the W. African coast, a species 

 ha\'ing' an extremely close affinity with L, toiiiipes. 



