ANIMALS ALLIED TO TRILOBITES. 297 



Trilobites, consists in there being a fully developed series of 

 crustaceous legs and antenna? in the Serolis (PI. 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 

 [Gymnohranchia,) in which the antennae become very small, 

 or altogether fail ; and that the legs being transformed to 

 soft and perishable paddles {pattes,) bearing branchiee, (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 PI. 45, Figs. 1. 2.) The history of this genus is im- 

 portant, on account of its relations, both to the existing and 

 extinct forms of Crustaceans ; it has been found fossil in 

 the Coal formation of Staflbrdshire 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 PI. 45, Figs. 1. 2.) there are but faint 

 traces of antennse, 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 branchis, and at the same time acting 

 as paddles for swimming. The same disposition of laminated branchie 

 is found also in the Serolis, Fig. 7. e. Fig. 8. is a magnified representa* 

 tion of these laminated branchiae, very similar to those at Figs. 3, e. and 



5.6. 



Thus while the Serolis (Fig. 7.) presents a union of antennae and 

 crustaceous legs with soft paddles bearing the Branchiae, we have in the 

 Limulus (Fig. 2,) a similar disposition of legs and paddles, and only 

 slight traces of antennse ; in the Branchipus, (Figs. 3 and 5,) we find an- 

 tennse, but no crustaceous legs; while the Trilobite, being without an- 

 tennae, and having all its legs represented by soft paddles, as in Branchi- 



