Trilobites, consists in there being a fully developed series of crustaceous legs and antennae 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.[1]
- ↑ In the genus Limilus (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 all 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 branchiæ is found also in the Serolis, Fig. 7. e. Fig. 8, is a magnified representation 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 antennas 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 antenna; in the Branchipus, (Figs. 3 and 5,) we find antennæ, but no crustaceous legs; while the Trilobite, being without antennæ, and having all its legs represented by soft paddles, as in Branchi-
