ANIMALS ALLIED TO TUILOIIITES. 393 
Ijites, consists in there being a fully developed 
series of crustaceous legs and antennae in the 
^erolis (PI. 45, Fig. 7.), whilst no traces of either 
these organs have yet been discovered in con- 
nexion with any Trilobite. M. Brogniart explains 
^he absence of these organs, by conceiving that 
the Trilobites hold precisely that place in the 
elass Crustaceans {Gymnohranchid), in which the 
nntennae become very small, or altogether fail ; 
nnd that the legs being transformed to soft and 
perishable paddles (pattes), bearing branchiae, 
(er filamentous organs for breathing in water), 
^ere incapable of preservation. 
A second approximation to the character of 
f rilobites occurs in the Limulus, or King crab 
(Lamarck, T. 5, p. 145.), a genus now most 
^bundant in the seas of warm climates, chiefly 
*a those of India, and of the coasts of America 
(see PI. 45, Figs. 1 . 2.) The history of this genus 
^s important, on account of its relations, both to 
the existing and extinct forms of Crustaceans ; 
tt has been found fossil in the Coal formation of 
Staffordshire and Derbyshire ; and in the Ju- 
fassic limestone of Aichstadt, near Pappenheim, 
together with many other marine Crustaceans of 
^ higher Order.* 
^ In the genus Limulus (see PI. 45, Figs. 1. 2.) there are but 
traces of antennse, and the shield (a.), which covers the 
^''terior portion of the body, is expanded entirely over a series 
small crustaceous legs (Fig. 2. a.). Beneath the second, or 
® doniinal portion of the shell (c.), is placed a scries of thin 
