MOLLUSCA. LAMELLIBRANCHIA 213 



Pinna. Shell generally thin, with coarse prismatic structure 

 (fig. 95), equivalve, inequilateral, wedge-shaped, without ears. 

 Umbones sharp, anterior, terminal. Valves truncate and gaping 

 posteriorly. Hinge-line straight, long. No teeth. Ligament linear, 

 almost entirely internal, lodged in a groove. Posterior adductor 

 large, sub-central ; anterior adductor close to the umbo. Devonian 

 to present day. Ex. P. hartmanni, Lias ; P. ajfinis, London Clay. 



Gervillia. Shell obliquely elongated, very inequilateral, 

 slightly inequivalve, the left valve a little more convex than the 

 right ; umbones almost terminal. Hinge straight, with broad 

 margin on which are numerous perpendicular, widely-separated 

 ligament-pits ; with two or more oblique ridge-like teeth. Ears 

 indistinctly limited from the rest of the shell, the anterior very 

 short, the posterior long. Posterior adductor imj>ression large, sub- 

 central. Trias to Eocene. Ex. G. subla?iceolata, Lower Greensand. 



Inoceramus. Shell variable in form, circular, oval, or oblong ; 

 inequilateral, inequivalve, ventricose or compressed, with ears indis- 

 tinctly limited. Umbones prominent, rather anterior. No teeth. 

 Surface with concentric (or rarely radiating) furrows. Hinge-line 

 straight, usually long, with numerous parallel, close-set, transverse 

 ligament-pits. Adductor impression rarely visible. Inner layer of 

 shell thin and nacreous ; outer layer thick, formed of large prisms. 

 Lias to Chalk ; common in Upper Cretaceous. Ex. /. concentricus, 

 Gault ; /. brongniarti, Chalk. 



Perna. Shell nearly equivalve, inequilateral, compressed, sub- 

 quadrate or sub-circular. Umbones at the anterior end. Hinge- 

 line straight, without teeth ; hinge-margin broad, with numerous 

 transverse, elongated ligament-pits placed close together and parallel 

 with one another. Right valve with a byssal sinus. Adductor im- 

 pression large, sub-central, double ; pallial line simple. Posterior 

 ear often large, not distinctly limited. Trias to present day. Ex. 

 P. mytiloides, Upper Jurassic ; P. epkippuim, Recent. 



Pteria ( = Avicula). Shell oblique, inequilateral, inequivalve, 

 left valve more convex than the right. Interior nacreous. Hinge 

 long, straight, with one or two small cardinal teeth and a lamellar 

 lateral tooth. Posterior ear wing-like and longer than the anterior. 

 A byssal sinus under the right anterior ear. Area small. Ligament 

 long, partly internal, partly external, in a groove. Posterior 



