558 



ZOOLOGY 



SECT. 



APPENDIX TO CRUSTACEA 



Class TKILOBITA. 



The Trilobita are extinct Arthropods peculiar to and characteristic of the 

 Palaeozoic rocks : 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 are 

 preserved only in exceptionally favourable cases. 



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

 regions, the head (c.^k), the thorax (th), and the abdomen (p), all of which usually 



''< 



A 



Fio. 441. Dalmanites socialis, dorsal aspect; H, the same' rolled up; C, under-side of 

 head of Phacops fecundus. r..xA. cephalic shield; > eye ; f.c. fixed cheek ; /.s. frontal 



suture ; <//. glabella ; ////. labruni ; mc. movable check ; p. pygidium ; pi. pleura ; *./'/' 

 sub-frontal plate ; tit. thorax. (After Gerstaecker.) 



present an elevated 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 shield (c.xh), the elevated median region of which, known as th( 

 t/lafic/fa ((jl), iisually presents three or four transverse grooves, probably indicat- 

 ing the presence of four or five segments. The lateral regions of the carapace 

 are divided by an oblique line of separation, the facial suture (f.s), into an innei 

 or mesial portion, the fixed cheek (f.c}, continuous with the glabella, and an 

 outer free portion, the mo cable, check (/.<) : the latter bears the large paired 

 <'"in]M.und eye (e). In some cases there is an indication of a dorsal organ, like 



