GENUS PISIDIUM.

This genus is separated from Cyclas to receive the smaller species, which have inequilateral shells, and have only one siphonal tube; the incurrent aperture being confluent with that for the passage of the foot (see fig. 3, p. 32). They have much the same habits as Cyclas, and live in similar situations. It is very difficult to dis

tinguish the species from each other, which, therefore, necessitates detailed descriptions of each. The largest and one easily recognized is

PISIDIUM AMNICUM (inhabiting rivers) (Pl. III., fig. 11).—The shell is triangular, deeply grooved concentrically, of a whitish-grey or pale brown; umbones very little produced; four lines long, one and a half thick, and three wide. It usually buries itself in the mud, but climbs by the aid of its long flexible foot among the aquatic plants. It is common, and universally distributed over Great Britain and Europe. It is fossilized in the newer Tertiaries.

PISIDIUM CINEREUM (Pl. IV., fig. 21) approaches the last, as regards size, more than any of the other species. Shell greyish, with one or two broad bands, more compressed and oval than the others of the genus, finely striated; umbones obtuse and prominent, sometimes slightly capped,

like Cyclas lacustris. Length three lines, thickness one and a half, and width two and a half. In ditches and slow streams widely diffused.

PISIDIUM PULCHELLUM (Pl. IV., fig. 24) is the smallest of the genus, differs from the last in size, and is of a less triangular form. The shell is only one and a half lines long and wide, half a line thick, of a glossy white, sometimes greyish, finely and irregularly striated.

This handsome and well-marked species well merits its specific name (pulchellus, small and beautiful); it is universally distributed. It inhabits stagnant and running water, and at the same time and place may be found on submerged plants, and buried in the mud. It occurs in the newer Tertiaries.

Professor Macgillivray made the following observations on individuals of this species found in a ditch near Aberdeen :-"When advancing in the water, the animal opens its valves a little, places itself erect by means of the foot, which it gradually protrudes until it extends to a length and a half of the shell, but often to twice its length. When thus extended it is of a linearoblong form, very little flattened, narrowed but rounder at the end. It then contracts, and drags the shell quickly forward; after which it is again extended, and again contracts. It is not always stretched out in a direct line, but is

moved in an undulating manner, often from side to side, and appears to act as a tentaculum, as well as an organ of motion. The siphonal tube, which is at the same time extended and kept so, is short, cylindrical, truncate, and changes but little; sometimes, however, it is nearly elliptical; it never extends beyond a sixth of the length of the shell. A current is seen passing out of it, and minute dark particles frequently escape. In

this manner the animal advances with considerable speed by jerks. At other times it ascends to the surface, where it proceeds in the same manner, with the shell reversed. The animal can advance equally with the shell inclined to either side; it can creep in any direction, on a level or inclined surface, and ascend or descend a perpendicular plane. On opening an individual in which young were seen, I found six lodged there. They were very large, much compressed, elliptical, with the umbones nearly central; the colour white; the surface beautifully glossed, and almost perfectly smooth. On being re

moved and placed in the water, some of them soon began to move in the same manner as the adults, but with the shell inclined on one side."

PISIDIUM HENSLOWIANUM (Pl. IV., fig. 19), named after Professor Henslow, is very closely allied to the last, which may be a variety of this,

but easily known by the plate-like projection on each valve near the umbones.

It occurs in several localities in England and in South Wales; the only Irish locality recorded is Finnoe, co. Tipperary. It occurs in the newer Tertiaries.

PISIDIUM NITIDUM-(the Shining Pisidium) (Pl. IV., fig. 22).- Shell suborbicular, very shining, finely and regularly striated, with a few separate and deeper grooves around the obtuse and subcentral umbones. Length one-twelfth of an inch, width one-fifth less. The most obvious distinctive characters are its rounded outline, glossy and iridescent appearance, the strong concentric grooves on the umbonal region, which are more readily seen in young shells, and the funnel-shaped siphon with its plaited outer margin.

It is somewhat generally distributed in lakes, ponds, and pools. Mr. J. Thompson found it abundantly in a cold turfy deposit conveyed by a mountain stream to a pond near Belfast, and on Utricularia vulgaris, growing in stagnant pools. These places are of a very different nature, the pond at the former being supplied with clear spring water, and at an elevation of 600 feet above the sea; the latter but a few feet above it, and supplied only with rain

water.

PISIDIUM PUSILLUM—(the Dwarf Pisidium) (Pl. IV., fig. 23).—The yellowish-white shell is oval, less compressed than the last, very finely but irregularly striated; the umbones nearly central; the posterior a very little more produced than the rounder anterior side. It is the least inequilateral of the Pisidia. It is the most common of the genus; is universally distributed throughout the country, inhabiting ponds, drains, ditches, &c. It does not apparently require a constant supply of water, being often found in marshy spots in company with and adhering to the same stones as the land mollusca inhabiting such places. Mr. Thompson procured it in the North and South of Ireland, among moss which was kept moist only by the spray of waterfalls. It is found in the newer Tertiary deposits.

PISIDIUM OBTUSALE (Pl. IV., fig. 20).-The shell of this species is closely allied to that of P.pusillum in its blunt, nearly central umbones, but is smaller and much more ventricose; the umbones are rather prominent and very blunt. It is one of the rarest of the genus, but occurs throughout Great Britain. Mr. J. Jones took it in immense numbers from a shallow pond on the hill above Mitcheldean, in the direction of the Wilderness, of which it appeared to be the only molluscous inhabitant.

Corbicula consobrina, closely allied to the