PART II. ATRACHIA. OSTRACIDiG. 389 



OsTREA Linn. Shell foliaceous, irregular, generally 

 attached by the under valve ; hinge usually without 

 teeth. 



Pedum Brug. Shell hatchet-shaped, depressed ; in- 

 equivalve, slightly auriculate ; umbones small, re- 

 mote ; sides of the lower valve turned upwards, with 

 a deep sinus for the passage of the substance by 

 which it adheres ; a small, central, spatulate tooth in 

 each valve bears the cartilage, and a transverse cen- 

 tral furrow between the umbones holds the ligament, 

 which spreads also between the umbones. 



P. spondyloideum. Sow, Gen. f. 1 — 5. 



Dendrostrcea Sw. Shell irregular, equivalve, attached 

 by its lower valve by extraneous processes ; the 

 margins solid and plicated. 



D. folium. Sow. Gen. f. 3. crista-galli. Sow, Gen, f. 2. 

 carinata. lb. f, 1. 



Ostrcea Linn. Shell irregular, round or oval, inequi- 

 valve ; the margins thin and brittle ; no teeth ; um- 

 bones central. 



O. edulis. Sow. Gen, f. 2. cingulata. lb. f. 1. 

 Virginica. lb. f, 1, 



Gryphcea Lam. Shell regular, 

 inequivalve ; one valve con- 

 vex, with the umbo recurved, 

 the other smaller, and nearly """^^^j^^^^^j^ 

 flat; margin of the valves not 

 foliaceous. 

 G. incurva (^^r.iso.) Sow, Gen. f. 2. dilatata, lb, f. S. 



Plicatula Lam. Shell irregularly ovate, plicated, in- 

 equivalve ; one valve more convex than the other ; 

 cardinal teeth %■ sagittate ; the sides strongly, regu- 

 larly, and transversely grooved ; between the teeth is 

 the internal ligament, (^fig. I29. e.) 



P. gibbosa. Sow. Gen, f. 1, 2. 



Anomia Linn. Shell thin, perlaceous, very irregular, 

 inequivalve ; attached by a calcareous mass, which 



