294 CRUSTACEA. TRILOBITA 



bordered by a flattened or concave margin which in 

 Trinucleus is very broad and highly ornamented. The 

 posterior angles of the cheeks, known as the genal angles 

 (h), may be rounded (e.g. Calymene), but are often pointed 

 or produced into spines, the genal spines (e.g. Paradoxides, 

 fig. 130). Each cheek is usually divided into two portions 

 by a suture (the facial suture, d) ; the inner part — 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 (/), is slightly movable on the fixed 

 cheek. The course of the facial suture varies in different 

 forms : it may commence on the posterior border inside 

 the genal angle (fig. 132), or at or near the genal angle (h), 

 or on the lateral border in front of the genal angle ; it 

 passes inwards to the eye and then bends forwards, and 

 may be continuous with the suture of the other cheek in 

 front of the glabella, or it may cut the anterior margin of 

 the head-shield, in which case it is sometimes united with 

 the suture of the other side on the inferior surface of the 

 head (fig. 125, d). When the sutures are continuous in 

 front of the glabella it is evident that the cheeks will also 

 be continuous. Since the position of the facial suture 

 varies in different genera the relative sizes of the fixed 

 and free cheeks will obviously vary too ; thus in Illcenus 

 the free cheek is very narrow, in Phillipsia very broad. 

 Owing to the fusion of the fixed and free cheeks the facial 

 suture is sometimes absent (e.g. some species of Acidaspis)\ 

 this is probably also the case in Agnostus, Microdiscus 

 and a few other genera ; but according to Beecher a facial 

 suture is present in Agnostus and is either at the margin 

 or on the ventral surface of the cephalic shield, and the 

 free cheek is then on the ventral surface ; this view, 

 however, is not accepted by Lindstrom and Holm. 



