308 CRUSTACEA. TRILOBITA 



Cheirurus. Head-shield semicircular, genal angles pointed 

 or with spines ; glabella convex, oblong or ovoid, with three pairs of 

 furrows which are sometimes continuous across, the last pair uniting 

 with the neck-furrow. Facial sutures continuous in front and 

 ending on the external margins. Free cheeks small ; eyes promi- 

 nent. Thorax with usually 11 segments, pleurae grooved, and 

 produced into spines. Pygidium small, with 4 segments, lateral 

 lobes with backwardly-directed spines. Tremadoc to Devonian. 

 Ex. C. articulatus, Devonian ; C. bimucronattis, Bala to Ludlow 

 Beds ; C. juvem's, Bala Beds. 



Deiphon. Glabella globular without furrows. Fixed cheeks 

 forming two long curved spines. Thorax with 9 segments ; pleurae 

 in the form of free spines. Pygidium short, prolonged into two 

 spines on each side. Llandovery and Wenlock. Ex. D. forbesi. 



SphaereXOCllUS. Glabella large, spheroidal, with 3 pairs of 

 furrows — the two anterior indistinct, the posterior curving back- 

 wards and joining the deep neck-furrow. Cheeks small ; eyes small, 

 near the axial furrow ; facial suture starts from the genal angle. 

 Thorax with 10 segments ; pleurae without grooves, with rounded 

 ends. Pygidium small, with 3 segments. Ordovician and Silurian. 

 Ex. S. mirus, Wenlock Limestone. 



Staurocephalus. Glabella with a spherical lobe projecting 

 in front of the cheeks ; the remainder of the glabella narrow and 

 cylindrical with 2 pairs of furrows and a deep neck-furrow. Cheeks 

 very convex, with a flat border. Facial suture starts from the 

 lateral margin and cuts the front margin. Eyes on stalks. Thorax 

 with 10 segments ; pleurae ridged, produced into spines. Pygidium 

 small, of 4 segments, with pleurae produced into spines. Bala to 

 Wenlock Limestone. Ex. S. mwchisoni, Wenlock Limestone. 



Encrinurus. Head-shield covered with tubercles ; with a 

 flat border, and pointed genal angles ; glabella pyriform, confluent 

 with the border in front, its furrows indistinct or absent ; eyes 

 small, on short peduncles. Facial sutures continuous in front, 

 ending just in front of the genal angles. Free cheeks narrow. 

 Thorax with 11 similar segments, pleurae ridged. Pygidium narrow, 

 triangular, with many segments in the axis, with 6 to 12 pleurae 

 bent backwards and diverging from the axis. Bala to Upper 

 Ludlow. Ex. E. punctatus, Wenlock Limestone. 



