CRUSTACEA OF THE PAST 259 



The head-shield shows indications of being com- 

 posed of five segments, and bears a pair of sessile 

 compound eyes. It is followed by a number (up 

 to twenty-six) of free somites, and the body ends in a 

 tail-shield, or "pygidium," which is often plainly com- 

 posed of several somites fused together. Although 

 Trilobites are among the commonest and most 

 familiar of fossils in the older rocks, the nature of 

 their appendages remained quite unknown until 

 within recent years, when specimens of several 

 species showing the structure of the limbs and 

 under-side of the body were discovered in America. 

 From these it appears that the head bore in front 

 a pair of long thread-like antennae and four pairs 

 of two-branched appendages, each with a jaw pro- 

 cess, or "gnathobase," turned towards the mouth, 

 which is covered below by a large anterior lip, or 

 " hypostome." It seems probable that the five pairs 

 of head-appendages correspond respectively to the 

 antennules, antennae, mandibles, maxillulae, and 

 maxillae, of Crustacea ; but the second pair appear 

 to have acted as jaws, retaining the gnathobase which, 

 among Crustacea, is only hinted at by the hooked 

 spine on the antenna of the nauplius larva. 



Each of the free somites and of those forming the 

 tail-shield bears a pair of two-branched appendages, 

 not differing greatly from the posterior appendages 

 of the head, but becoming smaller and more flattened 

 towards the hinder end of the body. The numerous 



