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 charactered, and therefore easily distinguished, the straight pillar p being a uniform mark, which at once separates them om the Helices, which their forms most resemble. Noing can exceed the beauty' and delicacy with which ome of these shells are marked, or the rich tints of color ith which others are stained. It is both recent and ssil. Extinct Fossil Shells. Such shells as have not been und in the recent, or living state, are considered as exnct. It is obvious that this criterion must, however, in e present state of knowledge, be exceedingly uncertain, nce further investigations most probably will show that any species now considered as lost, will be found still ving. Had all the shells unknown to Linnæus, been nsidered extinct, a great proportion of those now in the binets of collectors, would have been among the lost umber. Still there is, perhaps, no other rule by which aturalists can be guided, than to consider every unfound Fig. 92. species of shell, as well as phalous or headless animal, attached by a root-like base, ther substances. This base was jointed as seen in the re, and on its top were placed five wedge shaped bones, ch constitute the smooth circular central part, around ch are disposed five other pieces, called ribs. On the er edge of these are placed bones forming two arms, n of which divide into articulated fingers, and tentacula, eelers, which, when closed, bear a lily-like appearance, in the figure,) and when expanded, form nearly a cirar net of jointed meshes. These on closing, would are the prey and direct it into the stomach, which, Dably, was situated in a central cavity, at the upper of the base. These animals appear to have had conrable range for the seizure of their prey, without possing absolute locomotion, the peculiar mode of articuon, affording them a great degree of mobility, with siderable security against dislocation. These remains are found in that species of limestone led oolite, at Stonefield, in England, and are often in h abundance, that a considerable proportion of the rock bears to be formed of them. No living analogue of this animal is known. g. 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 26* med by the English geologists. It is found in the limeones of different names and ages, and also in chalk and ay. No living analogue of this genus, has ever been scovered. ig. 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, ere indefinite periods, and yet it is a shell of great comexity, showing that the most simple organizations, do not cessarily belong to the lowest strata. Fig. 96. Nautilus. A maný 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 bular aperture, running across the chambers, so as to m a complete siphunculus, fig. 96. To observe the chambers and siphuncle, the shell must sawn into two parts longitudinally. It is both fossil d recent. The name Nautilus signifies sailor, and under this term as formerly included the Argonauta, which, from its pposed skill in sailing. gave name to the whole. The great difference in the internal structure of the ells have induced subsequent conchologists to separate e genera. The present Argonauta is the true sailor, ough the credit is popularly given to the "Nautilus" ich it is said, "taught men how to sail." There is a considerable number of species of this genus, th fossil and recent, some of which are so small as only be defined by means of a microscope, while others are arly a foot in diameter. Fossil Insects. Insects, owing to the delicacy of their ucture, and the nature of the substance which enters o their composition, appear, generally, to be unfitted to in those changes which convert animal substances into ls, and hence few insects have been found in this state. crustaceous coverings of the bodies and limbs, and the I wing-cases of a few genera, are the chief, and pers the only fossils which can be referred to this class. Fig. 97. The Trilobite, or what is called in England the Dudley Fossil, fig. 97, is considered by some an insect, but by others a bivalve shell. Its superior covering, the only part distinctly preserved, is oblong, ovate, convex, and marginated; the anterior, wider part, is gibbous, and furnished with two semilunar tubercular projections resembling eyelids; and posterior to, and on the inner side of each of these, are two nd tubercles. Adjoining to this part, commences a ies of scale-like transverse slips, so disposed, that the ee rows of these connected slips, form three longituditransversely divided lobes, gradually diminishing to lower termination. In some specimens, the fossil is arly globular, showing that the animal had the power coiling or folding himself together, like the millipede. is doubtful how this animal accomplished locomotion, ace it is not certain that any indications of legs and feet ve been observed. This insect fossil has been found ur or five inches in length, though the common size is uch less. It occurs abundantly in the organic limestone formation Trenton Falls, New York. Fossil Fishes. Mr. Parkinson says, that "The fossil emains of fishes are found in such various states, under uch different circumstances, and in the formations of such istant periods, as cannot but lead the zealous inquirer to xpect that he shall derive from their examination, informtion of considerable importance." The fish, in some specimens, are found nearly entire, he harder parts, "all in their natural situations, with their cales, and even skin preserved. In others, all the other parts are removed except the skeleton, the bones of which either retain their original relative situations, or have undergone considerable distortion, or even dislocation. In some instances, not only separation of these parts has ta place, but the greater part of the skeletoп nas peen rewed."-Organic Remains, 277. There is, probably, no class of animals, the remains of ich are found in the fossil state, that are capable of bereferred to so many living analogues as fishes. Acling to Mr. Lacepede, more than thirty Asiatic, Afri, and American species, have been found in the neighhood of Verona, in Italy. The most celebrated locality ossil fish is at Monte-Bolca, in Italy, and it cannot be bted, that the catastrophe, whatever it might have been, ich caused this vast accumulation of the finny tribes, st have been almost instantaneous, and that the fishes e not only suddenly deprived of life, but immediately rwards buried in the deposite where they are now d. This appears from the singular circumstance, that fish was found in the very act of swallowing another, ing apparently had not sufficient time before it died to go its prey. Now, when any fish (especially if fured with an air bladder) dies, it remains at the bottom he water for two or three days, it then rises to the sur, decays, and the bones sink to the bottom. Hence, if e time had elapsed between the death of this fish and burial, it would, instead of being caught in the earth, e rose to the surface of the water, and at least separafrom the victim it was about to swallow. This is ably the reason why fossil fish are so rare, even among ta known by other remains to have been deposited from sea. They first rise to the top of the water, after bedeprived of life, where they remain until the flesh so decomposed, that the bones separate, and are dis sed. We may, therefore, conclude, that in most, if not ll instances, where fossil fish are found, they must have I and been buried by some extraordinary catastrophe or vulsion. Besides the proof just adduced of the suddenness of the olutions which have entombed fishes, in almost all s, the remains have been found in postures indicating olent death. Thus, at Eisleben, in Thuringia, where e is a locality of ichthyolites, the fish are in every ible attitude, some of them three feet long, often lying heir backs, or recurved into constrained positions, their heads crushed and disfigured. The strata ening them is a marly slate, impregnated with copper and men. |