85 AN ACCOUNT OF TWO NEW CRUSTACEA FROM THE TRANSITION AND CARBONIFEROUS STRATA. AMONG all the relics of former worlds, there is, perhaps, none that has more exercised the ingenuity of both naturalists and geologists, in the determination of its original and perfect form, than the remains of a crustaceous animal belonging to the transition strata known by the various names of Entomolithus paradoxus, Trilobite, and Dudley Locust. The earliest appearance of this fossil is in the Llandilo Flagstone, from which it extends upwards, through most of the intervening strata, into the Fullers' Earth, according to Mr. Parkinson, which is a formation above the Lias, and beyond which it becomes perfectly extinct. It seems to have been almost the sole representative of the extensive class of crustaceous animals through all this numerous series of rocks, whose deposition (to judge from the thickness of some of the layers, which are above five hundred feet) must have occupied immense periods of time. Trilobites belong to the class of Entomostraceous crustacea of Cuvier, and to the order of Pacilopoda, so termed from the varied form of their locomotive apparatus, some of which serve for feet or swimming organs, and the others, being furnished with fringed appendages, perform the office of gills. The other Entomostracea, with which the Trilobites are associated in certain of the strata, belong to the genera of Cypris, Eurypterus, and Limulus, which last possesses considerable analogy to certain genera of Trilobites. To omit the hypotheses of other distinguished naturalists and geologists, as Cuvier, Audouin, Goldfuss, and A. Brongniart, &c., Dr. Buckland (Bridgewater Treatises, No. 6) has endeavoured, with great success, to elucidate the structure of the Trilobites by a reference to their affinities in the genera Serolis, Limulus, and Branchipus; and as one of the fossils I am about to describe is analogous, in some respects, to these animals as well as to the Trilobites, it will be necessary to give some account of them before instituting a comparison between them and the specimen. For if we assume the recent genus Serolis as the type of this class of animals, we appear to pass gradually from its more perfect structure to the rudimentary form, (at least, apparently such,) of the Trilobite, through the intermediate genera of Limulus and Branchipus. The Serolis (ibid., pl. 45, fig. 6), in size and general appearance, resembles a small Crab. It is furnished with antennæ or horns, and with two claws. Its eyes are placed upon its back, and, like those of most of the animals I am describing, resemble the com pound eyes of insects. Its body is composed of several plates folding over each other, which, like the joints of the Lobster's tail, admit of considerable motion. They are prolonged laterally, so as to form a serrated edge to the body on each side. Its legs correspond to the number of the plates, at least of the projecting ones; and beneath the tail are a number of fringed appendages, which perform the office of gills. The Serolis is found on both shores of the Atlantic. The next genus, Limulus, or King-crab (ibid., pl. 45, fig. 1), usually wants the antennæ, dorsal plates, and ribs of the Serolis ; Its body is formed of a shield, consisting of two plates jointed transversely-the anterior semi-circular, the posterior triangular and terminated by a pointed tail, and its edges serrated and set with six spines on each side. It has four eyes-two compound close to the division of the shield, and two simple eyes, lying more in front, close to the median line. Its antennæ are very small; but it has twelve legs, and ten paddle gills. Its habitation is confined to the warm seas of India and America. The third genus, Branchipus (ibid., pl. 45, fig. 4), is the animal with which many persons are familiar from its exhibition in the solar microscope, where its agility, voracity, and the extraordinary vibrations of its paddles and tail, while the body was at rest, were remarkably striking. It has no proper feet or legs, but its members, and even its tail, being fringed with gills, answer the purposes of organs of locomotion and respiration. It possesses antennæ, and inhabits fresh water. Lastly, the Trilobite, (see plate, fig. 1), imperfect as it has hitherto been found, presents some of the characters of the two former genera, and, according to Brongniart, there is great reason to believe, of the latter also. Thus, it has the jointed body of the Serolis, and the large anterior plate or shield of the Limulus, and in some species the tail; but it has neither antennæ, legs, nor gills, though M. Brongniart conceives that it did possess the paddle gills of the Branchipus. Mr. Parkinson also believes that he has detected the existence of legs in one specimen, and M. Goldfuss gives some sections of a Calymene in which not only the presence of a ventral as well as a dorsal plate is clearly established, but also that of certain members, but whether these are legs or paddle-gills is not so evident. This account in vol. xv. of the Ann. des Sc. Nat., seems to have escaped the attention of English geologists, for Dr. Buckland says decidedly, that no Trilobite has been found possessed of legs All these classes of animals are aquatic. or antennæ. Trilobites consist of a thin oval plate of calcareous matter of various sizes, from half an inch to seven inches long, closely set with minute tubercles similar to those on the shell of the Crab. This plate is formed of many portions, the anterior of which, containing the eyes, is of a semicircular or crescent shape, and is called the shield; and to it are jointed a number of other plates like those on the Serolis, which, being crossed by two longitudinal grooves, together constitute a three-lobed tail, from which the fossil derives its name. The edge of the shell is turned inwards all round on the under surface, and hitherto no abdominal plate or members have been discovered. The eyes have been rarely found fixed in the shield, as they drop out, leaving an appearance of gaping eyelids. They are crescent-shaped, with the convex surface directed outwards, and are compound, as in the above-named genera, consisting of about four hundred facets or ocelli. From the close resemblance in the structure of the eyes of these animals, Dr. Buckland takes occasion to remark upon the permanence of the laws of nature from the earliest periods of which Geology supplies us with the records, when the earth was not yet fitted for the habitation of any animals higher than reptiles in the scale of organization, to the present time, when the earth teems with beings made in the image of the Creator, and but little lower than the angels. In this neighbourhood shields and tails of Trilobites are very abundant in the thin upper layers of the Dudley limestone, but the finer and more perfect specimens are chiefly found in the lower and thicker beds. They are also met with in ironstone nodules at Coalbrook Dale. Trilobites have been divided, by M. Brongniart, into five genera, which M. Latreille has distributed into three groups, viz., reniform or kidney-shaped, T. agnostus; contractile or folded, T. calymene ; and extended or flat, T. asaphus, T. ogyges, and T. paradoxoides. Agnostus is of a semicircular shape; calymene rolls itself up like a Wood Louse, and its segments are not extended laterally; asaphus has a lengthened tail (see Bridgewater Treatise, pl. 46, fig. 11); ogyges has a long shield extended backwards on each side to a point (ibid., pl. 46, fig. 9); and paradoxoides has no appearance of eyes, and its plates are extended over the side, like those of Serolis (ibid., pl. 46, fig. 8). Such are the chief generic characters of the Trilobite family given by Cuvier (Règne Animal, t. iv.), but they are not very distinct; and Dr. Buckland has recently added Limulus (Agnostus?) to them by the name of Limulus trilobitoides (ibid., pl. 46o, |