814 PROF. G. C. BOURXE OX THE [ISTov. 17 



Saigon River in Cochin China, as far as 20 or 25 miles from its. 

 mouth, and must therefore be capable of living in fresh or at 

 least slightly brackish Avater. This species is a typical member of 

 the genus Kerita, having a thick shell with spiral costfe, a clen- 

 ticufate outer lip, and a granular operculum, and is remarkable as 

 being one of the few species with these characters which is 

 tolerant of both salt and fresh water, though there are several 

 species of Neritina, usuall}^ a freshwater genus, which live in 

 brackish water or are even marine. It seems probable, however, 

 that several species of Nerita are capable of existing in brackish 

 water, since many of them are recorded from bays at the mouths 

 of large rivers, and an observation made by Mrs. Longstaff — to 

 Avhom I am indebted for some Avell-preserved specimens of Xerita 

 2)Ucata Linn. — shows that fresh water is not fatal even to 

 exclusively marine species. Mrs. Longstaff attempted to kill 

 some individuals of this species by immersing them in fresh 

 water : they were apparently uninjured, but did not like the new 

 conditions and crawled up the side of the vessel in which they 

 were placed, fixing themselves round its lim, apparently ready to 

 withstand a considei-able sojourn in the air. 



This question of habitat, in fresh, brackish, or salt water, is of 

 some importance in considering the generic distinction between 

 JSferita and Xeritina. The species of the latter genus are mostly 

 inhabitants of fresh water, and some are found only at the 

 sources of streams, far away from the sea. The numeroi-s 

 European species, of which N. Jliwiatilis is the most familiar 

 example, are freshwater forms, but occasionally occur in brackish 

 or even salt water. Yet many of the tropical species are partly 

 or wholly marine, e. g. X. uahiensis Lesson of the Indian Ocean 

 and Polynesia. Quoy and Gaimard (35) found iV. auriculata Lam., 

 in the sea. Dr. Ed. von Martens (27) describes the following 

 S. American forms as "species submai-ina?." X. virginea is common 

 in brackish water, but var. listeri, of E. Nicaragua, is found 

 throughout the river San Juan, and also in Grey town harbour 

 in localities where the water is alternately brackish and fresh. 

 JV\ picta Sowerby from S. Panama was observed in abundance 

 on a mudbank covered at times with fresh water, and has been 

 described as strictly marine by C. B. Adams. X. viridis, not rare 

 in the Mediterranean and in the Caribbean Sea, also in the 

 Bermudas, is truly marine and lives on Zostera. From all of 

 vv-hich it follows that whereas Nerita is very rarely found in fresh 

 water, Neritina is much more easily accommodated to different 

 conditions of life. 



In what precedes, I have accepted the usual distinctions- 

 between the two genera founded upon the charactei's of shell and 

 opercvilum, but it is a question, as will be more clearly shown in 

 the sequel, whethei' these characters are of sufficient importance- 

 to afford generic distinctions between forms, the intei-nal anatomy 

 of which is, in nearly all respects, so similar as to be practically 

 indistingiTishable ; and the further question will be raised as to 



