136 MOLLUSCA. LAMELLIBEANCHIATA. 



of teeth on both valves, (4) in the presence of a ligament, 

 (5) in the umbones never being perforated for a peduncle. 

 The classification followed here is based on the presence 

 or absence of siphons, and on the adductor muscles ; this 

 is not altogether a natural one, but it is convenient for 

 palseontological purposes. There are two orders, (1) 

 Asiphonida, (2) Siphonida. 



OEDEE. ASIPHONIDA. 



Animal without siphons, mantle lobes free or united 

 at one point only, pallial line simple. 



Section 1. Monomyaria. 

 One adductor (the posterior) present only. 



Ostrea*. Shell irregular, lamellar, inequivalve, slightly 

 inequilateral, fixed by the left valve. Left valve convex, 

 with radiating ribs or striae, umbone prominent ; right 

 valve flat or concave, often smooth and without stria3 or 

 ribs. Ligamental cavity triangular or elongated. Hinge 

 without teeth. Adductor impression sub-central ; pallial 

 line indistinct. Trias (? also Carboniferous) to present 

 day. 



Gryphaea. Shell inequivalve, free in the adult. Left 

 valve large, with a prominent incurved umbone. Right 

 valve flattened or concave. Lias to present day. 



Exogyra. Shell fixed by the left (larger) valve. 

 Right valve flat, operculiform. Umbones more or less 

 spiral, directed posteriorly. Upper Jurassic to Chalk. 



* All the genera of Mollusca described are marine unless otherwise stated. 



