138 MOLLUSCA. LAMELLIBRANCHIATA. 



Section 2. Heteromyaria. 

 The anterior muscle small, the posterior large. 



Avicula. Shell oblique, inequilateral, inequivalve, left 

 valve more convex. Interior nacreous. Hinge straight, 

 with one or two small cardinal teeth. Auriculate, the 

 posterior ear, wing-like and longer than the anterior. A 

 byssal sinus under the right anterior ear. Hinge-area 

 small, striate. Ligament partly internal, partly external, 

 the internal in a deep groove. Posterior adductor im- 

 pression large, sub-central. Ordovician to present day. 



Pterinea. Shell inequilateral, oblique, equivalve, 

 auriculate, the anterior ear shorter than the posterior ; a 

 byssal sinus under the right anterior ear. Hinge-line 

 long, straight ; with anterior small teeth, and posterior 

 elongated, laminar, teeth. Ligamental area large, longi- 

 tudinally striated. Posterior muscular impression large, 

 shallow; anterior impression small, deep, below the an- 

 terior ear. Ordovician to Carboniferous. 



Gervillia. Shell obliquely elongated, slightly inequi- 

 valve, inequilateral, the left valve a little more convex. 

 Umbones almost terminal. Hinge-line straight, broad, 

 with numerous perpendicular widely separated ligamental 

 pits ; its posterior edge with two or more obscure teeth 

 diverging behind. Ears not very distinct from the rest of 

 the shell, anterior ear short, posterior long. Adductor 

 impressions as in Avicula. Trias to Eocene. 



Inoceramus. Shell variable in form, circular, oval, 

 or oblong, inequilateral, inequivalve, ventricose or com- 

 pressed, not auriculate. Umbones prominent. No teeth. 



