[479] “On a collection of shells from the Solomon Islands” by Edgar A. Smith. (Proceedings of the Zoological Society: June 2nd, 1885.) This paper is illustrated with two coloured plates of the new shells.
As an instance of the unexpected results, which may fall to the lot of others in this group, I may here add, that out of eleven land and fresh-water shells that I collected in the small island of Santa Anna, which is only 21⁄2 miles in length, four were new species, and besides there were some new varieties. The stations of these four species may be suggestive. Two of them—Helix (Videna) sanctæ annæ and Helix (nanina) solidiuscula—were generally found on the trunks of the cocoa-nut palms at the coast; whilst the other two occurred in situations far more likely to yield new species, Melania sanctæ annæ being obtained from a small stream in the interior of the island, and Melania guppyi being found dead in the stomach and intestines of a fish that frequents the fresh-water lake of Wailava. This last shell would appear to live in the deeper parts of the lake, as I only found one living specimen, all the others being obtained from the stomach and intestines of these fish. Mr. Smith describes it as “a very remarkable and distinct species.” Its length is about 11⁄5 inches; and its sharp-pointed spire was to be sometimes seen protruding through the vent of the fish, which evidently digests the animal and ejects the shell. These fish were usually 9 or 10 inches long; but the full-grown shells were found also in fish half this size, when the relation between the length of the shell and the size of the fish was truly alarming. Since the little fish actually swallow sharp pointed shells measuring a fifth of their own length and pass them out through the vent after they have digested the animal, we must credit them with a remarkable capacity for adapting their diet to circumstances.
To exemplify the variation which some species of shells display in this group, I will take the instance of Helix (Geotrochus) cleryi, Récluz. This species is probably distributed through the whole group; but considerable variation prevails in different islands. Amongst the several forms which I obtained, three were named as new varieties, var. meridionalis from Santa Anna, var. simboana from Simbo or Eddystone Island, and var. septentrionalis from the islands of Bougainville Straits, the localities of the two last varieties being only 80 miles apart. Mr. Smith remarks that this species is “subject to considerable variation in size, colour, and form, apparently resulting from difference of habitat. . . . . . . . . Whether these several varieties should take specific rank is questionable, for, although there is a considerable difference between the extreme forms, even in the series of nearly one hundred specimens under examination, the gradual transition from one form to another is observable.”
Amongst the more singular in appearance of the land shells, I may refer to the large Bulimus (B. cleryi) which I found on the north coast of St. Christoval. It attains a length of four inches. I was never able to get a living specimen, as they are said by the natives to live in the foliage of the high trees. The specimens which I obtained were empty shells which the natives of the Koofeh district on the north coast of St. Christoval are in the habit of throwing into heaps, each man when he picks up a shell throwing it into the next heap he passes. I was unable to learn the reason of this practice and the natives did not seem willing to tell me. . . . Two other Bulimi I commonly met with. One was the pretty Bulimus miltocheilus (Reeve), which, when the animal is young and the shell delicate, has a greenish-yellow hue resembling the colour of the leaves it feeds upon: as it grows older the shell becomes thicker and stronger, and in proportion as there is less need for protective resemblance, the greenish-yellow hue fades away, leaving a dull white colour behind. This species is found in St. Christoval and the adjacent islands. The other Bulimus (B. founaki, Homb. Jacq.) which I found in Faro Island, Bougainville Straits, and which had been only previously obtained at Isabel Island, attains a length of rather under three inches.
I come now to refer to the fresh-water shells of these regions. Stated in their order of frequency, the Neritinæ, Melaniæ, and Navicellæ are the common fresh-water shells of these islands. The Neritinæ were especially interesting to me. They abound in the streams: some of them preferring the moist rocks above the water, others finding their home in the waters of a quiet pool, whilst others, like the Navicellæ, prefer to buffet the full rush of the torrent. An important feature with reference to these fresh-water Nerites or Neritinæ 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 considerably wider distribution.” Thus, Neritina subsulcata (Sowerby) and N. cornea (Linné), are not only found in the Solomon Islands, but also occur in the Philippines: N. macgillivrayi (Reeve) and N. petiti (Récluz) 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 Neritina subsulcata[480]—a species which is also found as above stated in the Philippines, and at the same time is the most widely dispersed fresh-water Nerite 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 surprise to me, as I inferred from the result of Baron Aucapitaine’s experiments as related by Mr. Darwin,[481] that their close-fitting stony opercula 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 remembered that their calcareous egg-capsules, which are so commonly seen on the rocky sides of the streams, are 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 Navicellæ, and I have seen them on the backs of the valves of a Unio which I discovered in the Shortland Islands.
[480] In his paper Mr. Smith refers to the species experimented on as N. cornea: but in my own list he named a shell belonging to one of the Neritinæ in question as N. subsulcata.
[481] Cyclostoma elegans was the species tested: vide “Origin of Species,” p. 353, 6th edit.
One common feature of these fresh-water shells, whether Neritinæ, Navicellæ, or Melaniæ, 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 extensively eroded, particularly in the case of Neritina subsulcata, 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 rocks. In volcanic islands the erosion of the fresh-water shells is greater than in islands of calcareous formation; and in streams, which, like those of the north coast of St. Christoval, 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[482] that 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.
[482] “The Natural Conditions of Existence, etc:” London 1881: p. 212, circâ.
There are two common species of Neritina in these islands which I often confounded, viz., N. subsulcata and N. cornea; and I learn from Mr. Smith’s paper that these two species very closely approach each other. They, however, are usually to be found in different stations, N. cornea occurring on the trunks of palms and other trees away from the streams,[483] and N. subsulcata preferring the moist rocky sides of the streams a foot or so above the water.[484] Now and then they may be found encroaching on each other’s domain; for I have found 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 them together in the streams.[485] Now it is a significant circumstance, that the specimen of N. cornea in my collection which was found by Mr. Smith to make the nearest approach to N. subsulcata was one which I obtained from a stream in Choiseul Bay. It had, in this case, not only intruded on the station of N. subsulcata, but had also assumed some of the distinctive characters of that species. It, therefore, seems to me probable that a graduated series of the shells of these two species might be formed, which would present the stages of transition from the one species to the other. If this be possible, then I would suggest that the fresh-water Nerite (Neritina subsulcata) may have been transformed into the tree Nerite (Neritina cornea) in the following manner.