MOLLUSCA. LAMELLIBRANCHIATA. 139 



Surface with concentric (or rarely radiating) grooves. 

 Hinge-line straight, or slightly curved, elongated. Nume- 

 rous parallel close-set cartilage pits. Adductor impression 

 rarely visible. Inner layer of shell thin and nacreous, 

 outer layer thick, fibrous and prismatic. Trias to Chalk. 



Perna. Shell nearly equivalve, inequilateral, com- 

 pressed, subquadrate or sub-circular, auriculate. Hinge- 

 line straight, no teeth. Hinge-area large. Cartilage pits 

 numerous, parallel, close together and elongated. Right 

 valve with a byssal sinus. Adductor impression large, 

 sub-central, double. Posterior ear often large, anterior 

 rudimentary. Trias to present day. 



Mytilus. Shell equivalve, inequilateral, sub-triangular, 

 posterior border rounded. Umbones sharp, terminal, 

 anterior. Teeth small or obsolete. Ligament linear, 

 marginal, sub-internal. Anterior adductor impression 

 small, posterior large. Surface of shell with an epidermis. 

 Trias to present day. 



Modiola. Shell equivalve, inequilateral, oblong, in- 

 flated in front. Umbones obtuse, anterior, but not terminal. 

 No teeth. Ligament linear, marginal. Adductor im- 

 pressions like Mytilus. Devonian to present day. 



Lithodomus. Shell sub-cylindrical, very inequi- 

 lateral, rounded in front, wedge-like behind. Umbones 

 anterior, not prominent. Ligament internal. No teeth. 

 A thin nacreous layer. Carboniferous to present day. 



Pinna. Shell equivalve, inequilateral, wedge-shaped, 

 generally thin, not auriculate. Umbones sharp, anterior, 

 terminal. Valves gaping posteriorly. Hinge-line straight, 

 long. No teeth. Ligament linear, almost entirely in- 



