LAND AND FRESH-WATER SHELLS. 339 



Neritinoe is their wide dispersal. " Some of these species " — as Mr. 

 Smith remarks in respect of those in my collection — " range not only 

 through most of the islands of the Solomon Group, but have a con- 

 siderably wider distribution." Thus, Neritina siibsiilcata (Sowerby) 

 and N. cornea (Linne), a.re not only found in the Solomon Islands, 

 but also occur in the Philippines : N. macgillivi'ayi (Reeve) and N. 

 petiti (Recluz) alike exist in the Fiji and in the Solomon Groups ; 

 while N. porcata (Gould) has been found in Samoa and in Fiji as 

 well as in the opposite extremities of the group with which I am at 

 present concerned. Being interested in the question of the mode of 

 dispersal of these Nerites, I made the following experiment to test 

 their powers of sustaining submersion in salt-water. One individual 

 belonging to the species Nentina subsiclcata ^ — a species which is 

 also found as above stated in the Philippines, and at the same time 

 is the most widely dispersed fresh- water Kerite in the Solomon 

 Group — survived a submersion of twelve hours ; but not one out of 

 a dozen individuals was found alive after a submersion of five days, 

 although the water was changed from time to time. The result was 

 a sui'prise to me, as I inferred from tlie result of Baron Aucapi- 

 taine's experiments as related by Mr. Darwin,^ that their close-fitting 

 stony opercida would have enabled them to resist the action of salt- 

 water. Their death could have been scarcely due to want of food, 

 since I have kept shells, of this species for several months on a very 

 scanty diet, and since the powers of endurance of other fresh-water 

 shells are well known. The matter passed out of my mind until 

 after my arrival in England, when Mr. Smith put the question to me., 

 as to their mode of dispersal. I then lemembered that their calcare- 

 ous egg-capsules, which are so commonly seen on the rocky sides of 

 the streams, ai-e in all probability sufficiently thick to resist the 

 action of salt-water. Here is therefore a probable mode of dispersal, 

 and I see it is one which Mr. Smith refers to as such in his paper. 

 These egg-capsules " if attached to floating timber, might be carried 

 to considerable distances." They are often to be observed on the 

 outside of the shells of living Navicellce,z?[\^\ have seen them on the 

 backs of the valves of a Unio which I discovered in the Shortland 

 Islands. 



One cominon feature of these fresh-water shells, whether 



^ In his paper Mr. Smith refers to the species experimented on as N. cornea : liut in my 

 own list he named a shell belonging to one of the Neritince in question as N. subsulcata. 

 - Cydostor.a ekijans was the species tested : vide " Origin of Species,"" p. 3.")3, Gth edit. 



