180 BRACHIOPODA 



Cyrtia (fig. 71). Area on the ventral valve very large, with a 

 convex deltidium. Dental plates well developed but not joining. 

 Brachial skeleton as in Spirifer, but the apices of the spires are 

 nearer the hinge-line. Silurian and Devonian. Ex. C. exporrecta, 

 Wenlock Limestone. 



Uncites. Shell elongate-oval, biconvex, striated. Hinge-line 

 curved, no area. Umbo of ventral valve prominent and incurved, 

 often distorted ; peduncle-opening closed in the adult by a concave 

 deltidium. Dental plates strong. Apex of dorsal valve incurved 

 and partly hidden in the ventral valve ; cardinal process prominent. 

 Brachial skeleton spiral, apices of spires directed laterally. Devonian. 

 Ex. U. gryphus. 



Meristina. Shell biconvex, smooth ; hinge-line curved, no 

 area. Ventral umbo incurved in the adult, so as to conceal the 

 foramen. Teeth supported by dental plates which reach to near the 

 middle of the valve. Spires of brachial skeleton pointing laterally, 

 joined by a band bearing a median stem which is forked at its end. 

 Silurian. Ex. M. tumida. 



Athyris. Shell with transversely elliptical or sub-circular 

 outline and a median sinus ; the two valves nearly equally convex. 

 Surface often with concentric growth-lines produced into lamellae. 

 Hinge-line curved. Ventral umbo small, incurved, usually concealing 

 the peduncle-opening and deltidium ; with prominent teeth supported 

 by dental plates ; four muscular impressions. Dorsal valve with a 

 tube from the interior of the valve opening at the hinge. Brachial 

 skeleton consisting of two spires joined by a band; the apices of the 

 spires pointing laterally. Devonian and Carboniferous. Ex. A. 

 concentrica, Devonian. Seminula, Carboniferous, is allied to 

 Athyris. 



Atrypa (fig. 83). Shell sub-circular or oval, ornamented with 

 radiating ribs, often crossed by well-marked growth-rings. Ventral 

 valve convex near the umbo, depressed in front ; dorsal valve often 

 much inflated. Hinge-line short, slightly curved ; no area. Ventral 

 valve with a small circular foramen, a small deltidium, and two 

 strong crenulate teeth ; muscular impressions grouped at the centre 

 of the valve. Brachial skeleton formed of two spirals with their 

 apices directed towards the centre of the dorsal valve ; the two spires 



