TRILOBITA. 



373 



is distinctly trilobed ; the " limb " of the head-shield is com- 

 posed of two lamellae, and is perforated by numerous larger 

 or smaller foramina; and the "genal angles" are prolonged 

 into conspicuous spines which are usually single, but are 

 forked in T. Pongerardi. The glabella is prominent and 

 pyriform, with mere traces of lateral grooves, the facial 

 sutures being rudimentary, and the cheeks being tumid, and 

 generally furnished on each side with a small tubercle 

 seemingly representing the eyes. There are six body-rings ; 

 and the tail is triangular, with a distinct axis, and having 

 its margin entire and striated. The genus Dionide has a 

 sub-quadrangular glabella; and the " limb" of the head-shield, 

 though perforated, is rudimentary, and by absence of the 

 facial sutures becomes continuous with the cheeks. The 

 eyes are wanting, and there are six body -rings. In the 

 curious Lower Silurian genus Ampyx (fig. 231) the head 

 is sub-triangular, with spinose genal angles, and without a 



Fig. 230. Trlnudeus concentricus. Lower 

 Silurian. (After Salter.) 



Fig. 231. Ampyx midm. Lower Silurian. 

 (After Salter.) 



perforated " limb." The glabella is prolonged forwards in 

 front of the head-shield as a triangular process, which is 

 often extended into a long spine, while its lateral grooves are 

 obsolete, and eyes are wholly wanting. The facial sutures 

 are present and are discontinuous. There are five or six 



