ORDER ISOPODA ; TRILOBITES. 263 



800. The order Isopoda is probably the situation, in which 

 we are to place the remarkable fossils known under the name of 



A 



V 



B 



FIG. 487 CALVMKNK FIG. 488. A, ASAPHUS KXPANSUS. B, THE SAMK 



BiUMENBACHH. ROLLED UP. 



Trilobites-t from the three lobes into which the body is divided 

 lengthways (Fig. 487). 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. 488). Their exact situation, 

 however, cannot be known, until some information has been 

 gained in regard to their extremities, of which no remains have 

 yet been discovered. It is probable that these were very short ; 

 in which respect they would correspond with those of certain 

 parasitic Isopods; but they may have been soft and membranous, 

 and formed for swimming, like those of the Phyllopoda. They 

 bear no inconsiderable resemblance externally to the Chiton 

 among Mollusca (Fig. 584) ; and could probably, like them, 

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

 which they were adherent. Their Articulated character, how- 

 ever, is fully evidenced by the nature of their eyes, which have 

 the compound 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. 



