Fig. 65. e impression of this part, is represented by fig. 64. In the second, or the cortical part, the lines between the rhombs are of more breadth, the ridge broader, and less defined, and forms, with the contracted superior elevation, a protuberance, and the central part assumes the figure of a squamula. Fig. 65, represents the matrix or cast of this part. The third or ligneous configuration, differs extremely from the two former, and only close observation determines that it originates from the same plant. The ncillated appearance is here entirely lost; the surface is ghtly striated with a scarcely perceptible rising under e central ridge, and a minute, but distinctly raised dot in the place of the depression Fig. 66. in the epidermis. It has all the appearance of a peeled plant, which has been furnished with small branches, set in quincuncial order. This is represented by fig. 66. Eight species of this genus are described, but the plan of this work forbids further detail. In some instances, trees of large dimensions have been scovered in the fossil state, but these are mostly of the Monocotyledonous kinds, as the palms and tree-like erns. In the quarry of sandstone, belonging to the coal fornation, on which the city of Glasgow is built, the quarry. en came upon a tree in its place, and just as it had been rowing. The trunk is about twenty-six inches in diamter, not quite round, but somewhat oval, so that the north nd south diameter is several inches longer than the east nd west. The body of the tree is composed of sandstone precisely similar to the rest of the quarry; but the bark as been converted into perfect cherry coal, which adneres firmly to the stone tree, and renders it easy to remove the rock with which it is incrusted. About three Feet of the bottom of the tree has been uncovered; this 1 quarry of sandstone. The upper part of the trunk etrifactions. There is a popular opinion, that in the The appears to be, that as the wood decays, its place is lied by particles of stony matter, deposited from water; as those particles are exceedingly small, and the deof the wood slow, its fibrous structure is preserved in stone, after the wood has entirely disappeared. Wood r undergoes this change when in a state of soundness integrity; but only when it becomes spongy by decay, when all its constituents have disappeared except the dy or ligneous fibre. This is proved by most specis of petrified wood, which show a partial decay before process of mineralization began. here are two kinds of petrifactions; the one caused by infiltration of calcareous, and the other of silicious cles. alcareous Vegetable Fossils. Lime is not very fretly the mineralizing matter of vegetable fossils; it is, ever, sometimes found introduced into the remains of in the form of spar, or imperfect crystals; in the pact form, it is also found filling the interior of fossil and succulent plants. licious Vegetable Fossils. These are immediately nguished from the calcareous, by their greater hard the former giving fire with steel, while the latter are y scratched with a knife. The silicious fossils are arkable for the correctness with which the fibres and kings of the wood have been preserved. The color is generally grayish, or yellowish white, metimes passing into brown, and is easily broken into arp edged fragments. It is found in many parts of the orld, but the finest specimens are said to come from ungary. The English, Portland limestone, contains rge fragments of wood, petrified by silicious infiltration, e interstices often containing fine crystals of quartz. These petrifactions prove that silicious matter is soble in water, under ordinary circumstances, and that not only takes the form of the woody fibre, but also of ystals. Dr. Macculloch has shown also, that in many instances, e mosses and other small vegetables, become incrusted ith silicious matter, while in their vegetable state, and e thus preserved from decay. But these real cases, Lust be distinguished from the black, tree-like appearaces, which are often seen on the flat surfaces of limeone, and which are produced by oxide of iron, or man anese. Fig. 67. The vegetable matter is easily detected by mixing a little of the moss agate, ground fine, with some black oxide of copper,-exposing the mixture to heat, in a glass tube, stopped at one end, and bent so that the other may dip in lime water contained in a vial. If any vegetable matter be present, carbonic acid gas will form, and passing into the lime water, will give it a turbid or milky appearance. The adjoining cut represents a specimen of moss, apparently belonging to the genus hypnum, contained within a silicious deposite, called chalcedony. In some instances of this kind, the vegetable form is so perfectly preserved, that the plant seems to float as if in a liquid. Even the green cofor occasionally is preserved, and, in a few instances, the species has been determined. FOSSIL SHELLS. hat the student may understand what follows, it is nery for him to become acquainted with the principles which shells are arranged, and a few of the terms by ch the different parts of a shell are denoted. hells, in their recent state, are composed of carbonate me, mixed with a little animal or gelatinous matter. heir fossil state, the gelatinous matter is seldom prethough sometimes a small quantity has been de d. ofter cha situa CONCHOLOGY. onchology is the science which treats of the structure, gement, and properties of shells. Shells are inted by testaceous animals, and to which they are only ally attached. Crustaceous animals are confined ely within their coverings, each limb, or member g invested by its own peculiar shield, as in the loband crab. Many of the testacea are fixed by an atment to other substances, as the oyster and muscle; e others have the power of crawling along the botor of moving through the water, as the unio, (fresh r clam,) and the scallop. The animals which int shells are called mollusca or molluscous animals, the classification depends, not on the habits or form ne animal, but on the form and other properties of the 1. he Linnæan system of conchology, which is the most le of any that has been proposed, divides shells into tivalves, Bivalves, and Univalves. y valve, is here meant any single piece of shell, which s the habitation or part of the habitation of a molous animal. Any shell formed of more than two es is a multivalve. Bivalves consist of two distinct es, and univalves of a single piece. Fig. 68. 1. MULTIVALVES. This is much the smallest class, but contains some beautiful shells. The genus Lepas, which contains the common barnacle, fig. 68, belongs here. The Chiton or coat of mail is another member of this class. The generic description of Lepas is, "Shell multivalve; affixed at the base; valves unequal, erect."These shells are chiefly parasitic, being attached to extraneous substances, en to ships, pieces of wood, whales, &c. The generic racters of this class are derived from the number and ation of the valves. 2. BIVALVES. This class includes all such shells as composed of two pieces only, whatever their forms or hensions may be; and in these respects, the species difexccedingly. The two valves of the Chama gigas netimes weigh 500 pounds, and from this, there are all des of size down to that of a grain of sand. Fig. 69. b Bivalve shells, when their valves are similar in size and form are said to be equivalved; if not similar, inequivalved; when the anterior part agrees in form with the posterior, they are said to be equilateral, if not, inequilateral. The valves are connected at their 9 base by a ligament, with or without a hinge, the ligament being placed externally or internally. The belly, a, fig. 69, he most tumid part; the disk, b, is that part between the ly and the margin, c, which is considered to refer to the ternal side, or, as it may be termed, when the shell is aced on its base, the upper side; then the umbones (eminces) d, are beneath the hinge, and terminate in the Ents or beaks, e, which are incurved, reflected, or ear h a mad The heaks are frequently in particular shells a |