MYSIDACEA. 



By Walter M. Tatters aix. 



Dr. Annandale and Mr. Kemp have continued to send me further collections of 

 Mysidae made by them in various parts of the littoral of India, mainly in brackish 

 water. In the present paper I describe two new species, belonging to the genus 

 Gastrosaccus, and record a third one, Rhopalophthalmus egregius, Hansen (previously 

 known from Japan and the East Indies) for the first time from the coast of India, 

 where it appears to be an abundant form. The number of species of Indian brackish 

 water Mysidae is now raised to five and Dr. Annandale and Mr. Kemp are to be con- 

 gratulated on the success which has attended their work. My best thanks are due to 

 them for the opportunity of examining and reporting on these specimens. 



The majority of the specimens here recorded are from the Chilka Lake, a shal- 

 low lagoon on the east coast of India, some thirty miles long and ten miles broad, 

 connected with the sea by a narrow mouth. The salinity of the water in this 

 lake differs very greatly at different seasons of the year, 1 and Mr. Kemp informs me 

 in a letter that ''visiting the lake in September (1914) at a time when the water is at 

 its highest, we found that a considerable part of the lake, including the outer channel 

 as far as the sea-mouth, was filled with absolutely fresh water. A great part of the 

 fauna of the lake is thus able for some two or three months each year to exist in 

 perfectly fresh water, and many species exist in salinities ranging from fresh water to 

 water as salt as the Bay of Bengal (sp. gr. 10265)." Among such species are all but 

 one of the Mysidae here dealt with, and it is exceedingly interesting to find that they 

 have adapted themselves to such a changing environment. In connection with one 

 of the species, Potamomysis assimilis, I have suggested that the great changes in the 

 salinity of the sea-water may account for the relatively great variation in the shape and 

 armature of the telson, but in order to settle this question, a complete series of speci- 

 mens, taken regularly throughout the year, with notes on the salinity of the water 

 at the time of capture would be necessary. It is possible that the changes in sali- 

 nity are too rapid to allow of correlated changes in the structure of the species. The 

 other two abundant species, Macropsis orientalis and Rhopalophthalmus egregius, do 

 not show evidence of such variation. 



Of the five species of Mysidae now known from brackish water in India four occur 

 in the Chilka Lake. Three of these (Rhopalophthalmus egregius, Macropsis orientalis 



1 See Introduction, p. 5. 



