558 



ZOOLOGY 



SECT. 



APPENDIX TO CRUSTACEA 



Class TRILOBITA. 



The Trilobita are extinct Arthropods peculiar to and characteristic of the 

 ozoic rorks : they are specially abundant from the Upper Cambrian to the 

 Carboniferous. They are often found in a wonderfully good state of preservation, 

 owing to the hard exoskeleton covering the dorsal surface : the greater part of 

 the ventral region and the appendages were, however, very delicate, and arc 

 preserved only in exceptionally favourable cases. 



The body is depressed, more or less oval in outline, and divided into three 

 regions, the In ml (r.. -.//), the thorax (fh), and the ahdomen (;)), all of which usually 



/c 



><*" 



*-~ ir^ ^r *-MA 



-^i, - -=^*"i^^-,iv ^ 



^^ 1K 



B 



i^^^p; 



Ibv - 



i 441 ,-~ 1 i* lmanites socialis, .Inrsal aspect; B, the same rolled up; C, under-side of 

 head of Phacops fecundus. c.*7 t . cephalic shield; ,. eye ;/.-. fixed cheek; f.s. frontal 



; :<'. g'lal.ella II,,; labrum ; m.c. movable cheek ; ,). pygidium ; p?. pleura :./. 

 sub-frontal plate ; Ih. thorax. (After Gerstaec-ker.) 



promt an ch-vated median ridge and depressed lateral portions, whence the trilo- 

 bation generally characteristic of the group. The head is covered by a cara- 

 pace or cephalic Meld (c.sh), the elevated median region of which, known as the 

 glabella ( ; //i. usually j>r<---fnts three or four transverse -grooves, probably indicat- 

 ing tin- presence of four or nve segments. The lateral regions of the carapace 

 are divided by an oblique line of separation, the fit,-!,,/ suture (f.s), into an inn.T 

 or mesial portion, the fixed cheek (/.c), continuous with the glabella, and an 

 outer free portion, the moro.ll,- rhcvL- (m.c) : the latter bears the large paired 

 compound eye (e). In some cases there is an indication of a dorsal organ, like 



