212 ELEMENTS OF COMPARATIVE ZOOLOGY. 



abdomen are readily distinguished, and in which both thorax 

 and abdomen consist of an axial portion, and two lateral 

 regions or lobes, whence the name of the group. The 

 head bears a pair of compound eyes, a single pair of an- 

 tennae, and four pairs of appendages, which served at once 

 for walking and for taking food. Each segment of thorax 

 and abdomen supports a pair of two-branched appendages. 

 Trilobites appear in the earliest fossil-bearing rocks, and 

 the group died out soon after the period of coal-formation 

 (in the Permian). 



