same genus, Cyrena vanikorensis, M. Quoy observes: "Ne l'ayant pas trouvée dans les lieux marécageux, mais sur les bords de la mer, il est probable qu'elle vit à l'embouchure des rivières qui sont saumâtres à marée haute."*

The third class of cases, in which species of Mollusca that are generally found in the sea are taken in fresh water, is much more rare than the preceding. It is obvious that in such instances the animal must be possessed of the capability of adapting itself to the different characters of the two fluids. This capability exists in much more highly organized animals, such as fishes, many species of which constantly migrate from the sea and ascend the rivers to deposit their spawn; but in these cases it is the result of a regular and determinate habit, while in the Mollusca it appears to be entirely dependent on accidental circumstances.

In some marshes in the Island of Bourbon, in which the water is almost fresh, M. Rang has observed specimens of Aplysia dolabrifera in company with Neritina and Melaniæ.

The greater number of species of the genus Cerithium are truly marine, chiefly living in sandy bays, like our own Cerithium reticulatum. M. Lesson, however, found C. sulcatum, and Adanson the African species figured by him, in the pools of brackish water, sometimes overflowed by the sea, which are situated between the weeds and the belts of mangrove trees on the shore; and Mr. Say observes that the small species, called by him Pyrena scalariformis, but which is a true Cerithium, is found in great abundance in the fresh water of Florida Keys. He adds: "it is most certainly a freshwater shell, yet it is destitute of an epidermis."

The genus Bulla is also truly marine; but the Rev. Mr. Hennah some time since presented to the British Museum specimens of one of its species, resembling the Bulla hydatis, found by him in brackish pools on the coast of Chili; and Mr. Say has described a Bulla fluviatilis found by Mr. Aaron Stone deeply imbedded in the mud of the river Delaware.+

The Littorinæ, again, are all found either on the sea-shore or in the very brackish water of the mouths of rivers, except two, which, although described as Paludinæ by Pfeiffer and De Férussac, and formed into a distinct genus by Ziegler under the name of Lithoglyphus, agree with Littorina in every character of shell and operculum, and, as far as I can

Voyage de l'Astrolabe, tom. iii. p. 516.

See for this latter instance the Journal of the Academy of Natural Sciences of Philadelphia, vol. ii. p. 179.

ascertain from the descriptions, of the animal also. These are the Paludina fusca of Pfeiffer, and the P. naticoides of De Férussac: they are truly fluviatile.

These anomalies are not restricted to the univalves: bivalves have also their share. Thus, the genus Solen is generally and properly considered as marine; but Mr. Benson has lately discovered a species inhabiting the mud on the banks of the Ganges; and conceiving, from the nature of its habitation, that it ought to be separated from the common species, he has formed a genus for its reception under the name of Novaculina. On comparing, however, some specimens of the shell presented to the British Museum by Mr. Royle, I can scarcely distinguish it as a species from the Solen dombeyi of Lamarck, which is found on the coast of Peru; and I have two other species, very nearly related, one from the rivers of China, and the other from pools of brackish water on the coast of America. In like manner M. Nilsson has found his Tellina balthica, which appears to be little more than a variety of the Tellina solidula of our coast, in the brackish water of the shores of the Baltic. Avicula margaritifera, the mother-of-pearl shell, commonly found in the ocean, has been taken by M. Rang in marshes in the Isle of Bourbon, in the neighbourhood of the sea in which the water is nearly fresh. Specimens of Mya arenaria also are often found so high up the rivers that the water in which they live is brackish only during high tides. They are found, moreover, with freshwater shells on the coasts of the Baltic, while all the other species of the genus are found only where the water is quite salt.

in

By far the greater part of the species of Corbulæ are truly marine; but there is a large species of the genus, called by Dr. Maton* Mya labiata, brought with freshwater shells from the mouth of the Rio de la Plata; and this agrees many respects with the fossil Corbula gallica, which occurs in what are called the upper freshwater strata of the Isle of Wight.

The transitions to which the oysters intended for the London market are exposed may be mentioned as an additional illustration. Many of these are collected in the sea on the coasts of Guernsey and of France, and are brought to situations in the mouth of the river where the water is merely brackish during the ebb of the tide, and where they are consequently subjected to the alternate action of salt and brackish water twice in each day. It is even affirmed

* Linnean Transactions, vol. x. p. 326, t. 24, f. 3.

that oysters can exist in water absolutely fresh; for in the Museum of the Bristol Institution there is a large group said to have been dredged up in a river on the coast of Africa where the stream was so sweet as to have been used to water the ship. To these shells are attached specimens of Cerithium armatum; and the person by whom they were presented to the collection stated that Cardium ringens was found abundantly in the same situation.

The genus Cucullæa, again, is universally considered as truly marine; but Mr. Benson has found in the Ganges a small shell belonging to it, regarded by him as an Arca, but, on account of its freshwater origin, formed into a new genus under the name of Scaphula.

On this subject I may observe, that I was some time ago informed that Arca senilis was found in the rivers of Africa in company with Galatea radiata: M. Cailliaud, however, assures me that this is by no means the case, the shells in question being found near the mouths of the rivers, but never in the rivers themselves.

*

One of the most decisive facts regarding the finding of the same species of shell in both salt and fresh water is noticed by Say. Speaking of Theodoxus reclinatus, he observes, I found this species in great plenty, inhabiting St. John's river in East Florida, from its mouth to Fort Picolata, a distance of one hundred miles, where the water is potable. It seemed to exist equally well where the water was as salt as that of the ocean, and where the intermixture of that condiment could not be detected by the taste." The shell in question is determined, by specimens which I received from my late friend himself (to whom science is so deeply indebted, and especially for his researches into the zoology of North America), to be the Neritina meleagris, obtained in such abundance from the West Indian Islands. Nilsson too, as before mentioned, has noticed the Neritina fluviatilis, which in this country is not observed to inhabit ditches in the neighbourhood even of brackish water, living on the coasts of the Baltic, in brackish situations, in company with Lymnæa balthica and L. succinea; and M. Rang found Neritina auriculata in similar situations.

According to the observations of Olivier, the Ampullaria ovata inhabits Lake Mareotis, where it is taken in company with marine shells found also in the Mediterranean; and I have lately received (dead) specimens from

* Journal of the Academy of Natural Sciences of Philadelphia, vol. ii. p. 258.

the locality indicated. The same species was found by M. Cailliaud in freshwater lakes in the Oasis of Siwah, where it is called Bozue, and eaten as food. It thus appears to be found both in fresh and brackish water. Two of the species referred to this genus by Lamarck, his Ampullaria avellana and A. fragilis, are truly marine; but they differ from the others in animal and operculum, as well as in the sinuated form of the outer lip of their shell.

The common cockle of the shops, Cardium edule, is constantly to be seen in the ditches of brackish water in the neighbourhood of Tilbury Fort, which gradually become more or less fresh in proportion to the quantity of rain that falls between the periods of opening the sluices. It is to be observed that the specimens found in this situation are rather thinner and more produced posteriorly than those usually found in the sea. The species in question is also, according to Nilsson, found in the brackish water on the shores of the Baltic, but I am not aware whether or not it is there subject to a similar variation in form. Nilsson observes, however, that the marine species found in those localities are generally smaller than those found in other situations.

From this list of exceptions to the general rules which have commonly been regarded as decisive of the localities inhabited by recent shells, and of the nature of the deposits in which the fossil species are found, it is manifest that those rules cannot safely be made use of for practical purposes without considerable reservation.

J. E. GRAY.

296

LETTER XVII.

ON THEIR FOOD AND DIGESTIVE ORGANS.

IN reference to the present subject, I shall divide molluscous animals into three classes:-1st, those which take their food in a liquid form, or suspended in water; 2ndly, those which are more properly carnivorous; and, 3rdly, those which feed on vegetable matters.

To the first class all the Mollusca tunicata belong, and the tenants of the bivalved shells. There is no one of either of these extensive tribes which is furnished with any organ adapted to the capture or arrestment of prey, or with jaws or teeth to tear and masticate it; and, as the greater number are immovably fixed to one spot for life, or are only capable of such motions as raise or depress them in their furrows, they are necessarily content to await what moist nutriment is brought within reach of their lips by the waves and currents of the circumfluent waters. The Tunicata have the power of enlarging the capacity of their large branchial sac; and it is probable that, during this action, a portion of water rushes in, with all its contained animalcules and microscopic vegetables, which serve for the food of the individual. Sir J. G. Dalyell says that the food of the Ascidia "seems to consist of what may be eliminated from muddy solutions. Quantities of mud suspended in water are evidently absorbed and long retained; they visibly fill the intestinal cavities of those species whose transparence exposes the interior. If the clearest sea-water be rendered turbid, it is speedily purified by the secerning operation of internal organs, serving to select the nutrition, while the residue is rejected, to be discharged in rolls or cylinders." * What they select from this mud are the infusorial vegetables with which it is loaded. I have found in the stomach of the Ascidiæ, as also in the sac of some of the compound and smallest species (Alcyoneæ) myriads of very minute corpuscules, which the microscope showed to be Diatomaceæ of various genera.† Löwig and Kolliker found that the

*Rare and Remark. Anim. Scot. ii. 140.

+ Dr. Dickie has given the following list of Diatomacea which he found in the stomachs of different Ascidiae taken near Aberdeen.