40) 
M. Truneata, Lin 1111.—D, p, 42.—Lam. 1. 
—Ch. f. 1, 2.—Don. t. 92.—Pen. t. 41. f. 14.— 
Mont. p. 32.—Turt. B. p. 61.—F. p. 463.—Ovate, 
ventricose, truncated anteriorly, rounded posteriorly, 
ventral edge nearly straight; dull white under a tough 
yellowish wrinkled cuticle, marked with coarse striz of 
growth: tooth rounded obtuse entire and projecting 
forwards. 2...2,5. Common in chalk. 
M. Arenarta. Lin. 1112.—D. p. 42.—Lam. 2.— 
Ch. f. 3, 4.—Don. t. 85.—Pen. p. 79. t. 42.—F. p. 
463.—Mont. p. 30.—Turt. B. p.32.—M. Acutaand 
Mercenakia. Say.—Ovate, rounded at both extremi- 
ties but slightly attenuated and produced anteriorly, 
convex, thick, dull white under a wrinkled brown epi- 
dermis; inside glossy white; hinge composed of a 
rounded projecting tooth and a smaller acute one by its 
side. 3...5,5. Common. 
M. Nveecica. Turt. Lin.t. 4. p.178.—M. Strti- 
ATA. Mont. Lin. T. 11. p. 188. ¢. 13. f. 1.—Lyonsta 
S. Turt. B. p. 35. t. 3. f. 6, 7.—F. p. 463.—Oblong, 
thin, equivalve, semi-transparent, pearly white under a 
brownish epidermis, rounded at one end, waved and 
truncated at the other; tooth not fixed. 3...1. Rare. 
