Lepadidæ.—The pedunculated Cirripedes, of which the common Duck-barnacle is a well-known example, have a strong, muscular, hollow stalk, or peduncle, which supports a multivalve shell, containing the body of the animal. In Pollicipes and Scalpellum there are small calcareous plates covering the junction of the body with the peduncle. Detached valves of several species of these two genera are met with in the Chalk, Galt, and Shanklin Sand, of Kent and Sussex; and three species have been found in the English Tertiary beds.[435] The valves of these fossil Cirripedes are most usually found in a disconnected state ([Lign. 167], figs. 3 and 4); but the Tertiary Scalpellum quadratum is sometimes better preserved; and Mr. Morris has described a most beautiful group of Pollicipes (P. concinnus), attached to an Ammonite, from the Oxford Clay. Mr Wetherell, of Highgate, has also discovered, in the Upper Chalk, near Rochester, an almost perfect Cirripede, named by Mr. G. B. Sowerby, jun., Loricula pulchella ([Lign. 167], fig. 2), and lately more fully described in detail by Mr. Darwin.[436]
[435] See Mr. Darwin’s Monograph of the Fossil Lepadidæ, Palæontographical Society, 1851.
[436] Monograph of the Fossil Lepadidæ, p. 81, pl. v.
Crustacea.—The animals whose fossil remains we have now to consider, are characterised by their crustaceous external integument or shell, which is disposed in segments, more or less distinct, the annular portions supporting articulated limbs or appendages. They are aquatic, free, locomotive beings, and possess distinct branchiæ, or organs fitted for aquatic respiration. The Crab and Lobster are examples of those tribes in which the external crust is calcareous, and "coloured by a pigmental substance, diffused more or less irregularly through it; and is formed upon and by a vascular organized integument, or corium, which is lined by the smooth serous membrane of the visceral cavities." (Owen.) The subdivisions of this class have relation to the forms, combinations, and proportions of the primary rings or segments of the external crust or integument, but it will not be requisite for our present purpose to enter upon this department of the subject. It may, however, be necessary to mention, that in the normal type of Crustaceans, the integument consists of twenty-one rings or segments, which form the three regions into which the body is divided; namely, the head or cephalic, the thoracic, and the abdominal; each of which is assumed to consist of seven rings, although some of these are generally anchylosed, and form but one segment; and even the three regions are occasionally more or less blended together. The cephalic portion of the crust contains the principal organs of sense, and the commencement of the digestive apparatus, and includes the masticatory appendages. The thoracic portion is formed of the rings to which the extremities serving for locomotion are attached: and, together with the cephalic, contains almost all the viscera. The consolidation of the rings or segments takes place most generally in the cephalic, and next in frequency in the thoracic; and but rarely occurs in the abdominal region. These animals possess organs of sight variously modified, and in some species highly complicated; some have smooth or simple eyes (stemmata), and others compound eyes, like those of insects, with distinct facets. In one grand division (called Edriopthalmia), the eyes are sessile and immovable; in the other (Podopthalmia), they are supported upon moveable stems or peduncles. These few remarks on the organization of the recent crustaceans are required, to make our description of the fossil remains intelligible to the general reader. As the shell, or calcareous integument, even in those species in which it is very dense and thick, is moulded upon the soft parts it envelopes, the experienced naturalist is able, from its configuration alone, to obtain certain conclusions as to the form, size, and position of the contained viscera; and, as these animals annually shed their solid case and acquire a new one, which is moulded on the soft parts, the form and relative situation of the internal organs must necessarily be faithfully represented by the external integument, even when it has acquired its greatest degree of consolidation; thus the regions of the stomach, heart, branchiæ or respiratory organs, &c. may be distinctly traced on the external shell. Hence the fossil carapaces may afford important data regarding the structure and economy of the extinct species. M. Desmarest[437] was the first naturalist who successfully applied this phrenological method to the investigation of the fossil crustaceans.
[437] Histoire Naturelle des Crustacés Fossiles; par MM. Alex. Brongniart et Desmarest. 1 tom. 4to. Paris, 1822.
The fossil remains of Crustaceans consist of the calcareous covering or carapace, with the articulated extremities, and, rarely, the jaws and antennæ. For the most part, the specimens are mutilated, and present only portions of the carapace, abdominal segments, and detached claws; but in strata composed of very fine detritus, such as the cream-coloured limestones of Solenhofen and Pappenheim, examples often occur in the most beautiful state of preservation, appearing as if the animals had been carefully embalmed in a soft paste, that had quickly consolidated around them, and preserved them without mutilation or blemish. In some examples, even the colour of the original remains. The specimens found in hard limestones and coarse conglomerates are generally mutilated, and, as the under surface of the carapace, and the sternal plates to which the legs are attached, present more irregularity than the dorsal portion of the shell, they are firmly impacted in the stone, so as to render the development of some of the most important characters difficult, if not impossible. The antennæ and claws are often separated, or altogether wanting; the most common relics being the pincers and the carapace, or united cephalo-thoracic segments. The substance of the shell, which in the recent state consists of phosphate and carbonate of lime, with gelatine or cartilage, is commonly a friable carbonate of lime, tinged with oxide of iron. These remarks apply more particularly to the crabs, lobsters, shrimps, prawns, &c. Numerous species of the smaller crustaceans, as the Cypris, and the extinct family of Trilobites, occur in myriads, and, in some formations, are the principal constituent of deposits of great thickness and extent.
The remains of this class have been found throughout the vast series of the fossiliferous strata. Extinct forms appear in prodigious numbers in the most ancient formations, and are succeeded by genera which approach more nearly to the more highly organized crustaceans. The Crab and Lobster tribes are represented by certain species in the Lias, Oolite, and Chalk; while in many of the Tertiary strata the existing types prevail.
The London Clay, in the Isle of Sheppey, yields many beautiful examples of the higher order of crustaceans, as the Crab, Lobster, &c. In the Chalk these remains are more rare, but a few fine specimens have been obtained. On the Continent, certain localities are extremely rich in these remains. Upwards of sixty species were discovered by Count Münster in the Jura limestone, at Solenhofen; and the Muschelkalk of Germany has yielded several extinct genera. The beautiful state in which these fossils occur, is exemplified in the specimen from Solenhofen, figured in the frontispiece of this work; [Pl. I. fig. 2].
Fossil Crabs.—Of the brachyurous, or short-tailed, crustaceans, of which the common Crab is an example, and of the Anomura (abnormal-tailed), there are remains of several genera in the Tertiary deposits. The Isle of Sheppey is the most productive locality in England. The carapaces of several kinds occur in the septaria and nodules of indurated clay; the chelate hand-claws (pincer-claws) are often found detached, and sometimes in connexion with the shield. The most numerous specimens are referable to two species. One of them (Cancer Leachii) is from two to three inches wide, and has a convex shell, the surface of which is covered by minute punctations, with three tubercles on each anterior lateral margin.[438] The carapace of the other species is more distinctly lobed, and studded with aculeated or spiny tubercles; it is named C. tuberculatus.[439]
[438] Hist. Nat. Crust. Foss. pl. viii. figs. 5 and 6.