OF TRILOBITES. 



n 



the basal furrows (d) ; middle or ocular furrow (<?) ; upper or frontal furrow {/) ; with an 

 occasional pair of frontal furrows (/*). 



These furrows bound and include the various lobes, designated thus : the frontal lobe 

 («) ; upper or third lobe (i) ; middle lobe (h) ; basal lobe (g) ; neck-lobe (b). 



The sides or cheeks consist of a portion fixed to the glabella (Bj r k) ; and separated 

 from the free or moveable cheeks (o C) by the facial suture (q q). These moveable 

 cheeks bear the eyes ; the fixed cheeks include the eye-lobe (/•), the neck-furrow (c), and 

 in some cases the posterior angles (B), often produced into spines. 



The posterior margin (j) is usually divided by the facial suture, which, in other cases, 

 cuts the outer margin at q. 



The front margin of the head is sometimes produced into a point {k), and on its under 

 side (fig. 2) shows the rostral shield (m), with its suture (/) (rostral suture) ; the 

 inferior branches of the facial suture (s s), the incurved under margin (t), the labrum or 

 epistoma 1 («)■, and sometimes a cavity (P) for the reception of the ends of the pleurae in 

 rolling up. 



Each thorax-ring (fig. 3), in like manner, consists of the axis (A), with its articular 

 portion (b), divided by the axal furrows (A*) from the side-lobes or pleurae. These last, 

 whatever be their nature, are in one piece with the axis, never articulated with it. 



The pleura consist of a posterior portion (/), a fulcral or anterior portion (e), sepa- 

 rated by the deep pleural groove (cl). At c the fulcral points are seen, beyond which in 

 most genera the segment is facetted (a a), for rolling up. These facets {a a) are always 

 smooth, and slide under the preceding joint in the act of rolling. 



Sometimes the terminal portion is produced into spines, (gg). 



Fig. 3. 



Tig. 4. 



The tail, or pygidium (fig. 4), consists also of an axis (A), with its articular portion, as 

 in the thorax; the limb or lateral portion is either entire, as at (a*), or shows its component 



1 The exact nature of this piece is not quite certain. I use ' labrum ' in the descriptions. 



