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Contributions to Indian Malacology, No. VIII. List of Estuary shells collected in the delta of the Ibawady, in Pegu, with descriptions of the new species. By William T. Blanpoed, A.R.S. M., F. O. S., Cor. Mem. Z. S. &c;
[Received Hth November, 1866.]
A short visit to Calcutta, and access to various works on conchology which have, for some years past, been beyond my reach, have enabled me to prepare the following list of the species of mollusca collected by me in the Pogu delta during the early portion of 1862. In March and April of that year, whilst engaged in the Geological Survey of the country south of Bassein, I was compelled to traverse the net • work of creeks which intersect the Irawaddy delta in every direction, and, in so doing, I had many opportunities of searching for the various mollusca inhabiting the channels of brackish and salt water.
The western portion of the Irawady delta south of Bassein is of peculiar character. Instead of the endless alluvial flat which is usually alone met with near the mouth of largo rivers, the country is frequently undulating, and even, in places, hilly; the hills being surrounded by plains of alluvial soil intersected by tidal channels. Rock not unfreqnently oecurs in these creeks, and affords a habitat for many mollusca which are not met with in the usual muddy flats.
The Bassein river itself, one of tho numerous mouths of the Irawady, like the Mutlah and other great channels of the Ganges delta, is at present rather an arm of the sea than a river; as it receives no fresh water directly from the Irawady except during the height of the rains. In the cold weather the water is perfectly salt for many miles above the mouth, and marine animals abound. Thus for many days, during the time I was traversing the neighbourhood, the water swarmed with Medusa. The volume of fresh water which pours into the Bassein river can at no time be very large, for the mollusca which inhabit the southern side of Negrais Island, some distance within the month of the river, are typically marine, comprising species of Parmoj)ltorus, Triforis, TrocJius, Chama, &c, and not including any o( the usual estuary forms Assiminia, Amphilwla, Neritina, &a., whilst at Poorian Point and Pagoda Point, the two headlands which form the entrance to the Bassein river, precisely the same mollusca occur as along the Arakan coast near Cape Negrais.* At the mouths of those channels by which the mass of fresh water poured down by the Irawady reaches the sea, I do not think that any typically marine animals are met with, nor could they exist, for, in the height of the rains, I have found the water outside the mouth of the Rangoon river perfectly fresh and drinkable, and yet this is only a minor channel compared to the Chinabuckeer and the neighbouring branches, down which the great bulk of the water pours.
To the greater saltness of the Bassein river I attribute the presence of the numerous marine types mentioned in the following list. It will be seen that a few distinctly marine species were met with; the number, however, was small. There are also in the list two or three genera, forms of which do not appear to have been hitherto found in estuaries, e. g. Tectura, Sphenia, Scalaria; whilst, on the other hand, the genus Scaphula had previously only been met with in fresh water.
The fauna and flora of the Irawady delta appear to be twofold.f Farther from the sea, where the water is more or less brackish, the creeks are mostly narrow and deep, with steep banks, which are covered at high water, and bordered by an unbroken belt of salt swamp, in which grow high trees, chiefly of Bruguieria gymnorhiza f The views along the creeks, with their borders of dense high forest, are often of great beauty. This belt of salt swamp and high trees varies much in breadth, from a few yards to half a mile or more; inside it are either open plains, which, if uncultivated, are covered with high grass, or else rises, usually of gravel, occasionally of rock, which are covered with jungle.
The mollusks of this tract comprise Ncritina depressa, N. ohtusa and N. Smithii; the species of Tectura, Modiola, Martesia and Splteiiia named in the following list are met with wherever rocks occur j Scophula is found under stones, Auricula and Cyrena inhabit the salt swamp. Teredo perforates the dead trees. Neritina cornucopia is principally met with in this region, but is also found lower down the
* Amongst others, I fonnd spocies of Dolinm, Ricinula, Ranella, &c.
11 regret that my want of knowledge of botany and the paucity of the observations I was able to make upon the zoology, prevent mo from entering fully into this subject. I can merely point out the fact that a distinction exists between the fauna and flora of the delta neater to the coast, and that found furthor inland, and illustrate it in the single instance of the mollusca.
estuaries : Littorina melanostoma also occasionally occurs, but its home is nearer the sea.
Lower down where the creeks are broader, the belt of salt swamp is narrower in general, and a broad shelving muddy shore succeeds, the upper portion covered by a thick forest of Aviccnsia, while lower down Nipa palms frequently occur. The beauty of the wide creeks is greatly enhanced by the broad fringe of the bright green Auicensia, over the tops of which the summits of hills, covered with dense green forest, are frequently visible.* On the sloping muddy shore species oi Potamides, Assiminia, Amphibola, Plecotiema, Haminca, Stenothyra; Area granosa, Nassa planicostata, and Columbella Duclosiana are to be met with. On the stems of the Nipa and on the mangrovo bushes Xeritina crepidularia and N. cornucopia, Littorina melanostoma and L. scabra are found in abundance.
I have only included in the present list those shells from the Bassein river which are found above Negrais Island, for the reasons already stated. I regret that the list is not more perfect, and that I am obliged to leave a few specimens undetermined. On the other hand the majority have been carefully compared, and the names quoted may, I think, in most cases, be relied upon. Immediately after leaving Pegu, I was in England for a few months; and owing to the kindness of the late Mr. S. P. Woodward, of Mr. Arthur Adams, and especially of the late Mr. Hugh Cuming, who allowed me to compare my shells with the original types in his unrivalled cabinet, I was enabled to determine, not merely my estuary collections, but also a ranch more numerous series of marine species from the Arakau coast, in a manner which would have been simply impossible in India.
Unfortunately, during the years which have elapsed since these shells were compared, a few have been mislaid or lost during constant travelling in various parts of India. Still I hope that this list may have some value as a contribution to our knowledge both of the geographical distribution and of the habitats of mollusca. Several of the species named, and some of the genera have, so far as I am aware, never before
* So great is the height of the trees fringing tho upper creeks, and so completely do they shut out all tho surrounding country, that I was working amongst thorn for several days in ignorance of the existence of hills nearly 10U0 lee t higli witluu 15 or 2U miles of mo.
been shewn to inhabit the estuaries of India or Burma. Our estuarylists have hitherto been almost as imperfect as our catalogues of marine species; almost all that is known of the molluscan inhabitants of onr deltas being due to the labours of Mr. Benson, who has described many of the forms found in the Ganges.
It is, of course, highly improbable that the present list is nearly complete. Only a very small portion of the Irawadi delta was examined, and that imperfectly. Still the number of species is considerable, and probably includes all those which are most abundant. A few forms since found by Mr. Theobald and Mr. Fedden will be noted in their place.
The classification employed is mainly that of Messrs. H. and A. Adams, in the Genera of Recent Mollusca. I have not, however, followed those authors in employing the obscure and forgotten generic terms of Klein, Montfort and others I have only deviated from their arrangement in one essential particular, viz., the transfer of Assiminia from the vicinity of Helix, from which it differs in every point of structure, to that of Littorina, to which it is closely allied. If it be objected that Assiminia is as closely related to Cyclostoma as it is to Littorina, I can only suggest that Cyclostoma bo also relegated to the same position in the neighbourhood of Littorina.
Estuary of the Bassein river, creeping upon mud between tidemarks.
Not common. Found in the lower part of the delta, with the next species.
Family MITRID^E. No. 3, Columbella Ducloziana, Sow.
Found in abundance at one spot in the estuary of the Bassein river, amongst stones with mud. The specimens were unusually fine. I also met with this shell on the mud flats of Rumri Island, coast of Arakan, •
Family SCALAEIADiE (Scalida, H. and A. Adams.) No. 4, Scalaria, sp.
A minute species, apparently new. As I possess but a solitary specimen, which is not in the best possible order, I hesitate to describe it. It is one of the smallest forms known, measuring only 3 millimetres in length. It was found under stones in the Myittaya creek.
A single specimen was found at Port Dalhousie.
Both this and the last species are met with abundantly on mud between tide marks, not far from the sea. They also occur on the sea coast where it is muddy.
No. 8, Potamides (Telescopium) fuscus, Chemn.
Common on mud between tidemarks, where the water is completely Bait.
Besides the above, a species of Cerithidca has been found by Mr. Theobald in the estuaries of Burma. I did not meet with it.
Family LITTOKINIDiB. No. 9, Littorina melanostoma, Gray.
Very abundant upon " mangrove" trees, close to high water mark. No. 10, L. scabra, L.
Occurs with the last, which appears to pass into it. Two varieties of this form occur, one more coarsely sculptured and more stoutly keeled than the other. No. 11, L. zic-zac, Chemn.
Syn. L. undulata, Gray.
This species is frequently found on the sea coast, especially near months of rivers. In the estuary of the Bassein river, it occurs together with true estuarine forms. It is met with on stones and dead wood, close to high water mark.
A. small roundly ovate, dull red species, which occurs abundantly