ARTHROPODA. TRILOBITA. 197 



on the posterior border inside the genal angle, or on 

 the external border just in front of the genal angle ; it 

 passes anteriorly 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 

 often united with the other suture on the inferior surface 

 of the head. When the sutures are continuous in front of 

 the glabella, it is obvious that the cheeks will also be 

 continuous. Tn some genera (Agnostus, Trinucleus) the 

 facial suture is absent. The eyes (e) are placed on the 

 upper surface of the head, one on each free cheek near the 

 facial suture, and generally near the middle of the cheek ; 

 they are compound, each consisting of a number of lenses ; 

 in a few genera (Agnostus, Ampyx) eyes are absent. The 

 head-shield is continued on the under surface of the head 

 forming a marginal rim (fig. 54 a) ; attached to this in the 

 median line is a plate, usually oval in shape, situated just 

 in front of the mouth and known as the hypostome (6). 



The thorax (fig. 53 B) is formed of a series of segments, 

 which vary in number, from two to twenty-six ; these are 

 movable upon one another, in some cases sufficiently to 

 enable the animal to roll itself up like a wood-louse. 

 Each segment is divided into a central and two lateral 

 parts by means of two furrows. The central part is more 

 convex than the lateral and forms the axis (i), the lateral 

 parts being known as the pleurae (k). The anterior part 

 of the axis of each segment is not visible when the animal 

 is unrolled, since it is overlapped by the preceding seg- 

 ment, with which it forms an articulating surface. The 

 pleurae in some genera possess a longitudinal ridge, in 

 others a groove ; a few forms have smooth pleurae. Each 

 pleura at some distance from the axis is curved down- 

 wards and usually also backwards, the point where this 



