216 MOLLUSCA. LAMELLIBRANCHIA 



Hinnites : the young shell is like Chlamys ; the adult is irregular 

 like Ostrea, and is attached by the right valve. Cretaceous to present 

 day. Ex. H. cortesi, Pliocene. 



Pecten (restricted) : right valve very convex, left flattened. Ears 

 nearly equal. No byssal sinus. Cretaceous to present day. Ex. 

 P. maximus, Pliocene to present day. 



Neithea : similar to the last ; with numerous small denticles on 

 the hinge. Cretaceous. Ex. P. {Neithea) quadricostatus, Upper 

 Greensand. 



Lima. Shell obliquely oval, anterior part larger than the 

 posterior part, equivalve, compressed, with radiating striae or ribs. 

 Valves gaping anteriorly and sometimes posteriorly. Umbones 

 distant, sharp. Hinge-line straight without teeth, with unequal 

 ears. On each valve a triangular area, with a central ligament- 

 pit. Adductor impression large. Two small pedal impressions. 

 Carboniferous to present day. Ex. L. gigantea, Lias ; L. cardii- 

 fonnis, Middle Jurassic ; L. squamosa, Kecent. Sub-genera Plagi- 

 ostoma, Limatula, Mantettum, etc. 



Aviculopecten. Shell ovate, slightly inequilateral ; right 

 valve less convex than the left. Umbones distinct ; hinge-line 

 straight, long ; ears distinctly limited, the posterior larger than the 

 anterior and often wing-like ; a byssal sinus beneath the anterior 

 ear in the right valve. Hinge-margin with narrow, nearly parallel 

 grooves. A central pit for the internal ligament. Adductor im- 

 pression large, sub-central. Surface usually with radial ribs, and 

 concentric lines, the ornamentation different on the two valves. 

 Devonian to Permian. Ex. A. tabidatus, Carboniferous. 



Pterinopecten. Similar to Aviculopecten ; posterior ear not 

 distinctly limited ; both valves with the same kind of ornamentation. 

 Devonian and Carboniferous. Ex. P. papyraceus, Carboniferous. 



Ostrea. Shell with lamellar structure, irregular, inequi valve, 

 slightly inequilateral, fixed by the left (larger) valve. Left valve 

 convex, often with radiating ribs or stria? ; umbo prominent, some- 

 times directed anteriorly, sometimes posteriorly. Eight valve flat 

 or concave, often smooth. Ligamental cavity triangular or elon- 

 gated. Hinge without teeth. Adductor impression sub-central ; 

 pallial line indistinct. Trias to present day. Ex. 0. deltoidea, 

 Kimeridgian ; 0. edulis, Pliocene and Recent. 



