:U0 LAND AND FRESH- WATER SHELLS. 



Neritince^ Navicelloe, or Melanice, is the extensive erosion of the apices 

 and surrounding parts of the shells. In some instances I have 

 noticed that almost the entire exterior of the shell has been exten- 

 sively eroded, particularly in the case of Neritina subsitlcata, but I 

 always found that the erosion was greatest in non-calcareous districts, 

 where the free carbonic acid in the water is not all consumed in the 

 solution of the limestone rock^. In volcanic islands the erosion of 

 tlie fresh-water shells is greater than in islands of calcareous forma- 

 tion ; and in streams, which, like those of the north coast of St. 

 (Jhristoval, flow in the upper portion of their course through a district 

 of volcanic rocks and in the lower portion through a district of 

 calcareous rocks, the same difference in the degree of erosion may be 

 observed. I learn from a recent work by Professor Semper ^ tliat it 

 is the boring of a minute fungus which first exposes the calcareous 

 substance to the action of the carbonic acid, and that the mechanical 

 action of the stream in forming tiny whirl-pools in the cavity 

 probably assists in the erosion. 



There are two common species of Neritina in these islands which 

 I often confounded, viz., N. subsitlcata and N. cornea; and I learn 

 fi'om Mr. Smith's paper that these two species very closely approach 

 each other. The3'', however, are usually to be found in different 

 stations, N. cornea occurring on the trunks of palms and other trees 

 away from the streams,^ and N. subsitlcata preferring the moist rocky 

 sides of the streams a foot or so above the water.^ Now and then 

 they may be found encroaching on each other's domain; for I have 

 f(;und them together on the trunks and branches of areca palms and 

 tree-ferns in low lying moist districts, whilst, as at Choiseul Bay, I 

 found tiiem too-ether in the streams.* Now it is a sis^nificant cir- 

 cumstance, that the specimen of N. corjiea in my collection which 

 was found by Mr. Smith to make the nearest approach to A^. substil- 

 cata was one which I obtained from a stream in C^hoiseul Bay. It 

 had, in this case, not only intruded on the station of N. subsitlcata^ 

 but had also assumed some of the distinctive cliaracters of that 

 species. It, therefore, seems to me probable that a graduated series 

 of the shells of these tw<;. species might be formed, which would pre- 



^ " The Natural Conditions tf E.xistence, etc : " London 1881 : p. 212, circS,. 



- In St. Christoval I found tliis species on one occasion 1.50 feet above tlie nearest stream. 



*Tliis species often tnkes to the water. Some individuals that I kept alive on board used 

 to spend a quarter of an hour at a time in the water eating voraciously all the while. 



•» According to Prof. Semper, these two species in the Philippines live a large portion of 

 the year high up on the trees in mangrove swamps. (Ibid.) 



