126 MONOMYARIA. 



Placuna placenta. — The Placenta Placuna. Plate 

 XIV. fig. 3. Suborbicular, flat, white, and pellucid, with 

 fine longitudinal striae, slightly decussated. Six inches or 

 more in diameter. Inhabits the coasts of China, and the 

 Indian seas. 



** Shell foliaceous, frequently ponderous, with a semi-internal liga- 

 ment. 



Geiius 12.— YULSELL A.— Lamarck. 



Generic Character. — Shell longitudinal, subequivalve, 

 irregular, and free j umbones equal; hinge, with a projecting 

 callosity depressed above, and a conical obliquely arcuate 

 hollow for the ligament ; within each valve a lengthened 

 muscular impression. 



Vulsella Ungulata. — The Latchet Vulsella. Plate 

 XIV. fig. 4. Elongated, depressed, transversely striated ; 

 pale yellowish brown, with longitudinal imdulated darker 

 stripes. Four inches long. Inhabits the Indian ocean. 



Geiius 13.— OSTREA Linnceus. 



Generic Character. — Shell adhering to extraneous bodies ; 

 inequivalve and irregular, with the beaks separated, the 

 upper valve advancing as the animal enlarges ; hinge without 

 teeth ; the lower valve larger, concave, ligament semi-inter- 

 nal, fixed in the cardinal depressions, of which that of the 

 lower valve is sometimes prolonged, with the umbo, as the 

 animal advances in age. 



This genus has two sections ; * with a simple margin, and not pli- 

 cated, ** margins phcated. 



Ostrea edulis. — The Edible Oyster. Plate XV. fig, 

 3. Suborbicular, rugged, with undulated, imbricated scales, 

 and transversely striated ; one valve flat, and the other 

 convex; outside brownish-green ; inside pearly-white; ex- 

 ceedingly variable in form and size. Inhabits the coasts of 

 Europe. 



