196 ARTHROPODA. TRILOBITA. 



and the pygidium 1 ((7) ; each of these three regions is 

 covered dorsally by a calcareous shell. That covering the 

 head, and known as the head-shield or cephalic shield, is 

 semicircular or triangular in shape and is not segmented ; 

 in it may be distinguished a median and two lateral 

 portions ; the former is the more convex and is termed 

 the glabella (a), the latter are the cheeks. The glabella is 

 marked off from the cheeks by means of a furrow on each 

 side, known as the axal furrow (a'). The form of the 

 glabella varies in different genera, in some it extends 

 quite to the anterior margin of the head- shield, in others 

 only a part of the way; sometimes it is wider behind than 

 in front, at others vice versa, or it may be uniform 

 throughout. Generally it is divided at the sides into lobes 

 by means of transverse furrows (b), usually three on each 

 side ; in some cases the furrows from the two sides meet 

 in the middle of the glabella. On the posterior part of 

 the glabella there is another furrow, which extends quite 

 across, this is the neck-furrow (&'). The cheeks are each 

 more or less triangular in shape and less convex than the 

 glabella, they are frequently bordered by a flattened 

 margin. The posterior angles of the cheeks, often spoken 

 of as the genal angles (h), may be rounded (e.g. Calymene} 

 but are often pointed or produced into spines (e.g. Tri- 

 nucleus}. Each cheek is divided into two portions by 

 means of a suture (the facial suture, d). The inner part, 

 that is, that between the facial suture and the glabella, is 

 termed the fixed cheek (g) and is immovable ; the outer 

 part, known as the free cheek (f), is movable on the fixed 

 cheek. The course of the facial suture varies in different 

 forms ; it may commence at the genal angle (h), or 



1 These are by some authors regarded as cephalothorax, abdomen, 

 and post-abdomen respectively. 



