analogies of structure, which place them in near approximation to the inhabitants of the existing seas.* The anterior segment of the Trilobites (Pl. 46, a, passim,) is composed of a large semi-circular, or crescent-shaped shield, succeeded by an abdomen, or body (c,) composed of numerous segments folding over each other, like those in a Lobster's tail, and generally divided by two longitudinal furrows into three ranges of lobes, from which they have derived the name of Trilobites. Behind this body, in many species, is placed a triangular or semi-lunar tail or post-abdomen (d,) less distinctly lobated than the body. One of these Genera, the Calymene, has the property of rolling itself up into a ball like a common Wood-Louse. (See Pl. 46, Figs. 1, 3, 4, 5.) The nearest approach among living animals to the external form of Trilobites is that afforded by the genus Serolis in the class Crustacea. (See Pl. 45, Figs. 6, 7.†) The most striking difference between this animal, and the * See M. Audouin's Récherches sur les Rapports naturels qui extent entres les Trilobites et les Animaux articulés. †The Genus Serolis was first established by Dr. Leach, on the authority of specimens collected by Sir Joseph Banks, in the Straits of Magellan (or rather of Magalhaens, the proper name of the navigator, according to Captain King) during Sir Joseph's voyage with Captain Cook, and given by Sir Joseph to the Linnean Society; and of another specimen of the same Genus from Senegal given by Mr. Dufresne to Dr. Leach. From these Dr. Leach described and named the species represented in our plate; his description of this Genus is published in the Dictionnaire des Sciences Na. turelles, v. 12, p. 340. Captain King has lately collected many specimens of Serolis on the east coast of Patagonia, lat. 45. S. 30 miles from the shore, and brought up by dredging in 40 fathoms water; and also at Port Famine, in the Straits of Magalhaens, where it was thrown upon the beach by the tide; here Captain King saw the beach literally covered with them dead; he has observed them alive swimming close to the bottom among the seaweed; their motions were slow and gradual, and not like those of a shrimp; be never saw them swimming near the surface; their legs seemed-shaped for swimming and crawling on the bottom.. Trilobites, consists in there being a fully developed series of crustaceous legs and antennæ in the Serolis (Pl. 45, Fig. 7.,) whilst no traces of either of these organs have yet been discovered in connexion with any Trilobite. M. Brongniart explains the absence of these organs, by conceiving that the Trilobites hold precisely that place in the class Crustaceans (Gymnobranchia,) in which the antennæ become very small, or altogether fail; and that the legs being transformed to soft and perishable paddles (pattes,) bearing branchiæ, (or filamentous organs for breathing in water,) were incapable of preservation. A second approximation to the character of Trilobites occurs in the Limulus, or King crab (Lamarck, T. 5, p. 145.,) a genus now most abundant in the seas of warm climates, chiefly in those of India, and the coasts of America (see Pl. 45, Figs. 1. 2.) The history of this genus is important, on account of its relations, both to the existing and extinct forms of Crustaceans; it has been found fossil in the Coal formation of Staffordshire and Derbyshire; and in the Jurassic limestone of Aichstadt, near Pappenheim, together with many other marine Crustaceans of a higher Order.* * In the genus Limulus (see Pl. 45, Figs. 1. 2.) there are but faint traces of antennæ, and the shield (a.,) which covers the anterior portion of the body, is expanded entirely over a series of small crustaceous legs (Fig. 2. a.) Beneath the Second, or abdominal portion of the shell (c.,) is placed a series of thin horny transverse plates (Fig. 2, e. 2, e'. and 2, e",) supporting the fibres of the branchiæ, and at the same time acting as paddles for swimming. The same disposition of laminated branchia is found also in the Serolis, Fig. 7. e. Fig. 8. is a magnified representa" tion of these laminated branchiæ, very similar to those at Figs. 3, e. and 5. e. Thus while the Serolis (Fig. 7.) presents a union of antennæ and crustaceous legs with soft paddles bearing the Branchiæ, we have in the Limulus (Fig. 2,) a similar disposition of legs and paddles, and only slight traces of antennæ; in the Branchipus, (Figs. 3 and 5,) we find antennæ, but no crustaceous legs; while the Trilobite, being without an. tennæ, and having all its legs represented by soft paddles, as in Branchi A third example of this disposition, in an animal belonging to the same class of Crustaceans, whereby the legs are reduced to soft paddles, and combine the functions of respiration with those of locomotion, is afforded by the Branchipus stagnalis, (Cancer stagnalis, Lin.,) of our English ponds, (see Pl. 45, Figs. 3, e. 4, e. 5, e.) In the comparison here made between four different families of Crustaceans, for the purpose of illustrating the history of the long extinct Trilobites, by the analogies we. find in the Serolis, Limulus, and Branchipus; we have a beautiful example, taken from the extreme points of time of which Geology takes cognizance, of that systematic and uniform arrangement of the Animal Kingdom, under which every family is nearly connected with adjacent and cognate families. Three of the families under consideration are among the present inhabitants of the water, while the fourth has been long extinct, and occurs only in a fossil state. When we see the most ancient Trilobites thus placed in immediate contact with our living Crustaceans, we cannot but recognise them as forming part and parcel of one great system of Creation, connected through its whole extent by perfect unity of design, and sustained in its minutest parts by uninterrupted harmonies of organization. We have in the Trilobites an example of that peculiar, and, as it is sometimes called, rudimentary development of the organs of locomotion in the Class Crustaceans, whereby the legs are made subservient to the double functions of paddles and lungs. The advocate for the theory of the derivation of existing more perfect species, by successive changes from more simple ancient forms, might imagine that he sees in the Trilobite the extinct parent stock from which, by a series of developments, consecutive pus, is by the latter condition placed near Branchipus among the Entomostracous Crustaceans, in the order of Branchiopods, whose feet are represented by ciliated paddles, combining the functions of respiration and natation. At Pl. 45. Fig. 3. e. Fig. 4. e, Fig. 5. e, represent the soft branchia of Branchipus, performing the double office of feet and lungs. forms of more perfect Crustaceans may, during the lapse of ages, have been derived; but according to this hypothesis, we ought no longer to find the same simple condition as that of the Trilobite still retained in the living Branchipus, nor should the primeval form of Limulus have possessed such an intermediate character, or have remained unadvanced in the scale of organization, from its first appearance in the Carboniferous Series,* through the midway periods of the secondary formations, unto the present hour. Eyes of Trilobites. Besides the above analogies between the Trilobites and certain forms of living Crustaceans, there remains a still more important point of resemblance in the structure of their eyes. This point deserves peculiar consideration, as it affords the most ancient, and almost the only example yet found in the fossil world, of the preservation of parts so delicate as the visual organs of animals that ceased to live many thousands, and perhaps millions of years ago. We must regard these organs with feelings of no ordinary kind, when we recollect that we have before us the identical in * The very rare fossil engraved in Martin's Petrifacata Derbiensia (Tab. 45. Fig. 4,) by the name of Entomolithus Monoculites (Lunatus) appears to be a Limulus. It was found in Iron Stone of the Coal formation on the borders of Derbyshire. A similar fossil in the collectien of Mr. Anstice, of Madely, is engraved in our Plate 46", Fig. 3. In the Secondary period, during the deposition of the Jurassic limestone, the Limulus abounded in the seas which then covered central Germany; and it still maintains its primeval intermediate form in the King Crab of the present ocean. My friend Mr. Stokes has discovered, on the under side of a fossil Trilobite from Lake Huron (Pl. 45, Fig. 12.,) a crustaceous plate (f.) forming the entrance into the stomach, the shape and structure of which resemble those of the analogous parts in some recent Crabs. This organ forms another link of connexion between the Trilobite and living Crustaceans.-Geol. Trans. N. S. vol. i. p. 208, Pl. 27. struments of vision, through which the light of heaven was admitted to the sensorium of some of the first created inhabitants of our planet. The discovery of such instruments in so perfect a state of preservation, after having been buried for incalculable ages in the early strata of the Transition formation, is one of the most marvellous facts yet disclosed by geological researches; and the structure of these eyes supplies an argument, of high importance in connecting together the extreme points of the animal creation. An identity of mechanical arrangements, adapted to the construction of an optical instrument, precisely similar to that which forms the eyes of existing insects and Crustaceans, affords an example of agreement that seems utterly inexplicable without reference to the exercise of one and the same Intelligent Creative power. Professor Müller and Mr. Straus* have ably and amply illustrated the arrangements, by which the eyes of Insects and Crustaceans are adapted to produce distinct vision, through the medium of a number of minute facets, or lenses, placed at the extremity of an equal number of conical tubes, or microscopes; these amount sometimes, as in the Butterfly, to the number of 35,000 facets in the two eyes, and in the Dragon-fly to 14,000. It appears that in eyes constructed on this principle, the image will be more distinct in proportion as the cones in a given portion of the eye are more numerous and long; that, as compound eyes see only those objects which present themselves in the axes of the individual cones, the limit of their field of vision is greater or smaller as the exterior of the eye is more or less hemispherical. If we examine the eyes of Trilobites with a view to their principles of construction, we find both in their form, and in * See Lib. Ent. Knowledge, v. 12.; and Dr. Roget's Bridgewater Trea tise, vol. ii. p. 486 et seq. and Fig. 422–428. |