ANIMALS ALLIED TO TRILOBITES. 393 
bites, consists in there being a fully developed 
series of crustaceous legs and antennae in the 
Serolis (PI. 45, Fig. 7.), whilst no traces of either 
of these organs have yet been discovered in con- 
nexion with any Trilobite. M. Brongniart ex- 
plains 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 bran- 
chiae, (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 of the coasts of America 
(see PI. 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 Ju- 
rassic limestone of Aichstadt, near Pappenheim, 
together with many other marine Crustaceans of 
a higher Order.* 
* In the germs 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 
