TRILOBITE HUNTING. 205 



contained more living matter than was the case 

 with the others. (Fig. 55.) 



The Acidaspidce have from eight to ten body- 

 segments, the sides of which (pleuras) are turned 

 backwards. 



There are also the Bronteidce with large ex- 

 panded pygidium or tail, the Harpeidte with 

 horseshoe-like head-shield and numerously seg- 

 mented body, and lastly, the 

 head-shield of which is prolonged 

 into spines. 



It is from the structure of the 

 trilobites, moreover, that it has 

 been found possible to determine 

 the conditions of their existence 

 and their general habits. 



The chemical composition of the 

 carapace affords a sufficiently ac- 

 curate indication of the character 

 of the habitat of the creature ; as, Fl iel 

 for instance, the living and granu- shield of Phil- 

 lated covering of Pkillipsia, just i - psla " 

 referred to as living in the carboniferous seas. In 

 other cases the carapace seems to have been more 

 chitinous, like the wing-cases of insects. 



The fact that the body-covering is so often found 

 in separate portions makes it certain that trilobites, 

 like crabs, underwent the periodical process of 

 moulting. The head-piece or cephalic shield is 

 generally found alone, the thorax or ringed part 

 often so, and the pygidium also frequently occurs 

 separately. The moultings of Phillipsia are always 



