Some members of this genus might easily be mistaken for Murices or Buccina; but the Strombi have a depression, or sinus on the dilated wing, which is separate from the groove at the base of the shell, next the pillar. Attention to this, will lead to the distinction. It is both re

cent and fossil.

Fig. 86.

Genus Murex. Shell univalve, spiral, often formed with longitudinal membranous sutures; and beset with spines; aperture terminating in a canal, either straight or turned up backwards, and not inclining to the right or left, fig. 86.

The very peculiar form of the aperture or canal, is a very distinctive feature in this genus. This is oblongoval, or perfectly oval, and does not gradually contract into a canal, like the Strombi, and Buccina, but suddenly opens into it at the same or nearly the same width, which it retains through the whole length of the beak.

The famous Tyrian purple was extracted from an animal inhabiting one species of this genus. A single vein near the head contains the coloring liquor; but the art of dyeing, in latter times, has disclosed more beautiful, and much less costly colors than this produces.

The name Murex means rough or rock-like, a designation which fails to apply in many of these species. It is both recent and fossil.

Fig. 87.

Genus Turbo. Shell univalve, spiral; aperture contracted, round and entire. Fig. 87.

One of the best distinctions of this genus is the round aperture. The shells often closely resemble those of the Trochus genus, but, in these, the aperture is angular, often the only mark of distinction between the two genera.

The Turbo might at first be mistaken for the spire of another shell, but its unbroken base and round aperture will generally distinguish the genus. The name Turbo, means any thing which whirls around, as a top, in reference to the spiral form of the genus. It is both recent and fossil.

Genus Conus. Shell univalve and turbinate. Aperture effuse, longitudinal, linear, toothless, and entire at the

Fig. 88.

base. Columella smooth, base attenuated, sometimes marked with oblique grooves. Aperture sometimes dilated; whorls, mostly flat, often channelled, rarely crowned.

The great beauty of this genus, both in form and coloring, renders it highly interesting and valuable to the lovers of this science.

The rare species are sought after with avidity by shell collectors, and the most beautiful kinds often sell for considerable sums. The Conus gloria-maris, and the Conus cedonulli, sometimes bring from twenty to twentyfive guineas for single shells.

The name Conus, a cone, refers to the shape of the genus.

Fig. 89.

Genus Trochus. A spiral, sub-conical univalve; aperture four sided and somewhat angular, having the upper part of the margin converging towards the pillar, which is oblique. Fig. 89.

In some species the aperture tends to an oval form, but these are distinguished from the Turbines by a tooth-like projection. It

must, however, be confessed, that there is much difficulty in distinguishing some specimens of these two genera from each other. In general the Trochi have the form of a pointed cone, capable of standing nearly erect on their bases. The word Trochus has a similar meaning to Turbo-the common name is top shell, or button shell, the shape being similar to that of a common spinning top or an ancient conical button.

Fig. 90.

Genus Helix. Shell univalve, spiral, translucent, brittle; aperture coarctate, or contracted, lunate of circular, having the segment of another circle taken from the whole area, fig. 90. The common land snail is a good example of this genus. The whorls are contiguous, and the body of the shell always forms a lunate projection into the aperture, and this character will distinguish the Helices, from the Trochi and Turbines. Another mark of the genus is tenuity, or thinness and translucency.

Fig. 91.

Genus Nerita. Shell univalve, spiral; gibbous, flat underneath, aperture semiorbicular, or semi-lunar, having uniformly the pillar lip, or columella straight. Fig. 91.

The Nerita is a genus well characterized, and therefore easily distinguished, the straight pillar lip being a uniform mark, which at once separates them from the Helices, which their forms most resemble. Nothing can exceed the beauty and delicacy with which some of these shells are marked, or the rich tints of color with which others are stained. It is both recent and fossil.

Extinct Fossil Shells. Such shells as have not been found in the recent, or living state, are considered as extinct. It is obvious that this criterion must, however, in the present state of knowledge, be exceedingly uncertain, since further investigations most probably will shew that many species now considered as lost, will be found still living. Had all the shells unknown to Linnæus, been considered extinct, a great proportion of those now in the cabinets of collectors, would have been among the lost number. Still there is, perhaps, no other rule by which naturalists can be guided, than to consider every

Fig. 92.

unfound species of shell, as well as of quadrupeds extinct, though there is a thousand fold greater prospect of finding new species of the former, than of the latter.

Encrinite. This is a genus of singular and curious animals, which being found among the lower strata of rocks, are supposed, by those who think there were successive creations, to have been among the first inhabitants of this earth.

The Lily Encrinite, fig. 92, is a beautiful fossil, so called from its resemblance to the form of that flower. It was an

acephalous or headless animal, attached by a root-like base, to other substances. This base was jointed as seen in the figure, and on its top were placed five wedge shaped bones, which constitute the smooth circular central part, around which are disposed five other pieces, called ribs. On the upper edge of these are placed bones forming two arms, each of which divide into articulated fingers, and tentacula, or feelers, which, when closed, bear a lily-like appearance, (as in the figure,) and when expanded, form nearly a circular net of jointed meshes. These on closing, would secure the prey and direct it into the stomach, which, probably was situated in a central cavity, at the upper part of the base. These animals appear to have had considerable range for the seizure of their prey, without possessing absolute locomotion, the peculiar mode of articulation, affording them a great degree of mobility, with considerable security against dislocation.

These remains are found in that species of limestone called oolite, at Stonefield, in England, and are often in such abundance, that a considerable proportion of the rock appears to be formed of them.

No living analogue of this animal is known.

Fig. 93.

Fig. 94.

Belemites. This is a conical, spindle-shaped stone, of brown radiating spar, generally terminating at the small end in a point, and having at the larger end, a conical cavity, naturally retaining a conical testaceous body, divided into chambers, by plain concave partitions, and pierced by a siphuncle, or orifice, fig. 93.

This extinct fossil occurs in great abundance in the kind of limestone called lias, in several parts of England. It is also found in the newer limestone groups of this country.

Ammonites. A multiocular, or many celled, spiral shell, with contiguous apparent turns; the chambers being divided by winding partitions, and pierced by a siphuncle or winding orifice, placed at the outer side, fig. 94.

More than seventy species of this fossil are found, and have been determined and

named by the English geologists. It is found in the limestones of different names and ages, and also in chalk and clay. No living analogue of this genus, has ever been discovered.

Fig. 95.

Orthoceratites. A multiocular, and slightly bent, cylindrical, or slightly conical univalve shell; the chambers separated by a plain septa, concave towards the larger end, and pierced with a siphuncle, fig. 95.

This is considered one of the earliest creations, by those who suppose that the days of creation were indefinite periods, and yet it is a shell of great complexity, showing that the most simple organizations, do not necessarily belong to the lowest strata.

Fig. 96.

Nautilus. A many celled spiral univalve, the turns contiguous, the outer one including the others; the chambers separated by plain, or nearly plain partitions, placed transversely, and concave outwards; these are perforated by shelly tubes, connected by a tubular aperture, running across the chambers, so as to form a complete siphunculus, fig. 96.

To observe the chambers and siphuncle, the shell must be sawn into two parts longitudinaly. It is both fossil and

recent.

The name Nautilus signifies sailor, and under this term was formerly included the Argonauta, which, from its supposed skill in sailing, gave name to the whole.

The great difference in the internal structure of the shells have induced subsequent conchologists to separate the genera. The present Argonauta is the true sailor, though the credit is popularly given to the "Nautilus," which it is said, " taught men how to sail."

There is a considerable number of species of this genus, both fossil and recent, some of which are so small as only to be defined by means of a microscope, while others are nearly a foot in diameter.

Fossil Insects. Insects, owing to the delicacy of their structure, and the nature of the substance, which enters into their composition, appear, generally, to be unfitted to