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



the main area throughout the year can survive in a medium varying from fresh water 

 to water of specific gravity i-oi5o. 



The species of Decapoda found in the course of our investigations in the Chilka 

 lyake are listed on pp. 204, 205. Each species is classified biologically as a permanent 

 inhabitant, a seasonal immigrant, or a casual visitor; the range of salinity which it 

 can withstand in the lake is noted and indication is made of its habitat, whether 

 found in the main area or the outer channel, with a brief note on its further dis- 

 tribution. 



About 60 per cent of the total number of species are able to exist throughout 

 the whole range of salinities that occur. 



The jDermanent inhabitants, those that are found in the lake throughout the 

 year, form the most important part of the Decapod fauna and comprise the great 

 majority of the species obtained. Among them, as might be expected, are repre- 

 sentatives of a number of families known to inhabit brackish water. Such are the 

 Hymenosomatidae, Grapsidae, Ocypodidae, Geocarcinidae, Portunidae, Atyidae and 

 Penaeidae. Several of the genera met with were, however, known hitherto only 

 from the sea and precise indications of the salinity that the others are able to 

 endure has in most instances been lacking. The lycucosiidae and Xanthidae are 

 marine types whose presence in fresh and brackish water is unexpected and this is 

 also true of the majority of the Alpheidae', the Palaemonid genera Urocaris and 

 Periclimenes and the planctonic Sergestid, Lucifer. 



The only representatives of freshwater genera that can be classed as permanent 

 inhabitants are the two species of Caridina. 



So far as our present knowledge goes, the following species are confined to 

 estuarine tracts and lagoons, that is to say, to waters communicating directly with 

 the sea, but of low or variable salinity : — 



Ebalia malefactrix. Lcipocten sordidulum. 



Philyra alcocki. Clibanarius olivaceiis. 



Elamena [Trigonoplax) cimex. Upogebia (Upogebia) heterocheir. 



M acrophthalmus gastrodes. Periclimenes demani. 



Pachygrapsus propinquus. Ogyrides striaticauda. 



Alphciis paludicola. 



No Oxyrhyncha\ Dromiacea, Hippidea, Galatheidea, Palinura or Stenopidea 

 were obtained in the lake. Among the Brachygnatha it is singular that no repre- 

 sentatives of the Pinnotheridae and Gonoplacidae occur. Many of the species be- 

 longing to these two families are mud-dwellers and, judging from the account which 

 Miss Rathbun has given*, they constitute a very important part of the fauna of the 

 inshore waters of the Gulf of Siam. 



' One member of this family, Alpheopsis haugi, is indeed known from a freshwater lake in French 

 Congo (see Couticre, Bitll. Mus. d'Hist. nat. Paris, 1906, p. 37b). 



^ The family Hymenosomatidae, one species of which occurs in the lake, is sometimes included in 

 this tribe. I have followed Alcock in placing it in the Brachyrhyncha. 



» Rathbun, Danske Vid. Selsk. Skrijt. (7), Naiurvid. og. math., V, p. 302 (1910). 



