Mytilus as the Iridina do to the Uniones, as they both differ from their apparently allied genera by the adhesion of the lobes of the mantle, and the possession of a siphon; but in our present imperfect state of knowledge of the animals of the Conchifera, and of the value of the adhesion or the separation of these parts in a systematic point of view, perhaps it is better to regard them as the type of a distinct family.

The fossil genus Congeria agrees with Dreissena in many particulars, and perhaps belongs to the same family, if it is in reality a separate genus.

1. DREISSENA Van Beneden. (Dreissena.)

The animal of this genus differs from Mytilus in the mantle being closed, while in the latter it is open. In the latter, the retracting muscles are divided into several bundles, each of which has its proper attachment to the shell, while in Dreissena these muscular cords are united into a single bundle, which has only one point of attachment. In Mytilus, the branches adhere through their whole length; in Dreissena, the extremities are free, and float upon the posterior transverse muscle.

This genus was first established by Mr. Van Beneden. It has also been named Tichogonia by Rossmäsler in 1835. He did not know the animal, although it had been described eleven years before by Mr. Sowerby.

128. 1. DREISSENA polymorpha. Zebra Dreissena. Shell triangular, keeled, olive, varied with black, wavy-lined.

Mytilus Volgæ. Chemn. xi. 205. f. 2028. polymorphus. Pallas, Voy. Russ. App.

211.; Sow. Gen. Shell. f. 4. Mytilus Chemnitzii. Férussac.

lineatus. Waardenburg, Mol. Belgi.
Kickx, Monag.

arca.

? Volgensis. Gray, Ann. Phil. 1825.

Hagenii.

1826, 140.

Baer, Fér. Bull. Sci. Nat.

Mytilus Toreyi. Stenz.

Tichogonia Chemnitzii.

t. 3. f. 69.

Rossm. Icon. i. 1113.

Dreissena polymorpha. Van Beneden, Ann. Sci. Nat. 1835, 210. t. 8. f. 1-11.; Strickland, Mag. Nat. Hist. 1838, 361.

Inhab. lakes and rivers, attached to stones, timber, and other shells.

Mr. James de Carle Sowerby first brought the fact of this animal having been introduced before the public on Nov. 2nd, 1824, when he presented some specimens to the Linnean Society, stating them to

be "probably the Mytilus polymorphus Gmelin, 3363, which is found in abundance, attached to shells and timber, in the Commercial Docks, by James Bryant, Esq., who uses the animal as bait for perch." Mr. Sowerby observes, that "the strong resemblance which it bears to the marine Mytili is very remarkable. Independently, however, of the septa within the valves, there are many other differences to be observed, several of which are in the structure of the included animal, although it possesses a strong byssus; among others, the foot is small and the lips of the mouth are differently placed, being more like those in the animal of the Unio ovalis. It has two tubes, and the mantle is united almost all round, and bordered with a bright orange between two bands of black. Some of the septa within the beaks appear to be a kind of disease, as they are not constant."

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"The same species is found in the Danube and in the rivers of Russia; but the British species are much larger and finer than any foreign one I have seen.” (Linn. Trans. xiv. 585.)

In 1825, in a List of Shells not taken notice of by Lamarck (Ann. Phil. 1825), I stated that this shell would "perhaps form a genus distinct from Mytilus, and peculiar for its fresh-water habitation," and added that, "like Mollusca of that station, the animal can live for a long time out of water. I have

kept one for three weeks, when it was still healthy. It is found in the Commercial Docks, where it most likely has been introduced with timber from the Volga."

I am now confirmed in the idea that this is the way in which they were introduced, as a friend has

informed me that he has seen them sticking to the logs of Baltic timber before they were unloaded from the ship. (See Wiegmann, Arch. 1838.) In the dock they attach themselves to stones, Uniones, Anodons, and the walls of the docks, as well as to the logs.

This species illustrates how rapidly molluscous animals may become naturalised, and spread over a great extent of country; for Mr. J. de C. Sowerby, in 1825 (Zool. Journ. i. 584.), first recorded it as naturalised in the Commercial Docks, where he observed that it had probably been brought with the timber: it has since been widely extending itself, and is now to be found in most of the docks communicating with the Thames. In 1834, Mr. Stark communicated to the Wernerian Society the discovery of this species in the Union Canal, near Edinburgh; and in 1836, the Rev. M. J. Berkeley, the eminent cryptogamic botanist, discovered it, with Mr. J. Streatfield, on the piers of the bridge which crosses the Nen at Fotheringay; and again a little higher up the same river, on stones of a small overfall at Tansor: he believes they were introduced from Wisbeach on timber since 1828.

It has been naturalised into Holland and on the Rhine. It is also found with tertiary fossils in Transylvania, Moravia, and near Vienna.

Mr. Lyell (Geol.), not being aware that these animals had the power of living a long time out of water, and that they were most probably brought in the holds of ships with the Baltic timber, and thus introduced into our docks, where the timber is unloaded, believes that the animals were introduced attached to the bottom of Baltic ships, and thus obliged to pass through the sea, before being again brought to their

natural station in fresh water.

And Mr. Garner, in his curious but rather crude paper on the anatomy of Lamellibranchiata (Mag. Nat. Hist. n. s. iii. 303.), ventures to explain this theory by supposing that the animals "kept their valves constantly closed" during the voyage through the sea to the fresh water!

"O LORD, how manifold are thy Works! in Wisdom hast thou made them all: The Earth is full of thy riches."- PSALM Civ. 24.