214 MOLLUSCA. LAMELLIBRANCHIA 



adductor impression large, sub-central. Silurian to present day. 

 Ex. P. media, Barton Beds ; P. Mrundo, Recent. Sub-genera, 

 or closely allied genera, are Actinopteria, Leiopteria, Ptero?iites, 

 Oxytoma. 



Pseudomonotis. Similar to Pteria, but the shell is oval, 

 the left valve large and very convex, and the right valve flattened ; 

 the anterior ear small or rudimentary. Devonian to Cretaceous. 

 Ex. P. ecMnata, Cornbrash. 



Aucella. Shell thin, obliquely elongate, inequilateral, inequi- 

 valve, with concentric folds or ribs. Left valve convex, with 

 prominent incurved umbo ; ears indistinctly limited. Right valve 

 flattened, anterior ear triangular, with a deep byssal sinus ; posterior 

 ear indistinctly limited. Hinge-line straight, short, without teeth. 

 Ligament external. Upper Jurassic and Lower Cretaceous. Ex. 

 A. keyserlingi, Speeton Series. 



Pterinea. Form similar to Pteria ; left valve flattened. 

 Hinge with small transverse anterior teeth, and laminar posterior 

 teeth. Area large, with longitudinal grooves for the ligament. 

 Posterior adductor impression large, shallow ; anterior impression 

 small, deep, below the anterior ear. Silurian to Carboniferous ; 

 common in Devonian. Ex. P. Icevis, Devonian. 



Posidonomya. Shell thin, oblique, oval, equivalve, com- 

 pressed, with concentric furrows. Umbones small, sub-central. 

 Hinge-line straight, short, without teeth ; posterior ear compressed, 

 indistinctly limited. Silurian to Jurassic. Ex. P. becheri, Carboni- 

 ferous. 



Conocardium. Shell more or less trigonal, very inequi- 

 lateral, with radiating ribs ; posterior side short, truncated, forming 

 a cordate posterior end, produced into a long tube ; anterior side 

 oblique, compressed, wing-like, gaping. Umbones small, pointed, 

 incurved. Hinge-line long, straight. Ligament partly external, 

 partly internal, attached to a plate behind the umbones. Anterior 

 adductor impression large, deep ; posterior impression shallow. 

 Inner margins of valves toothed. The truncated end bearing the 

 tube is regarded by some authors as anterior, and the wing-like end 

 as posterior. The affinities of this genus have not yet been deter- 

 mined. Silurian to Carboniferous. Ex. C. Mbernicum, Carboniferous 

 Limestone. 



