ORDER ISOPODA ; TRILOBITES. 263 



874. This is probably the situation, in which we are to place 

 the remarkable fossils known under the name of Trilobites, from 



FIG. 595. CALYMENE FIG. 59C. A, ASAPHUS EXPANSUS. B, THE SAME 



BLUUENBACHII. EOLLED UP. 



the three lobes into which the body is divided lengthways (Fig. 

 595). These are found in the very earliest of the formations that 

 contain fossils at all; and they appear to have ceased to exist before 

 the creation of Mammalia. They resemble the Isopoda in the 

 equality of their segments, as well as in the tri-lobed division of 

 the body, which is presented by some of the recent species of that 

 Order ; and also in their tendency to roll themselves into a ball 

 (Fig. 596). Their exact situation, however, cannot be known, 

 until some information has been gained in regard to their ex- 

 tremities, of which we know as yet next to nothing. The slight 

 traces of these organs which have been discovered seem to in- 

 dicate that they bore some resemblance to the feet of the Phyllo- 

 poda, and it is probably as an Order intermediate between the 

 latter and the Isopoda that we must regard the Trilobites. They 

 bear no inconsiderable resemblance externally to the Chitons 

 among Mollusca (Fig. 673) ; and could probably, like them, draw 

 the border of the shell completely down to the surface on which 

 they were adherent. Their Articulated character, however, is 

 fully evidenced by the nature of their eyes, which have the com- 

 pound structure peculiar to that Sub-Kingdom. This has been so 

 perfectly preserved in many specimens, that the facets may be 

 counted with the aid of a magnifying-glass ; and as many as four 

 hundred have been found to exist in a single specimen. 



