History of British animals - University of Guam Marine Laboratory

428 MOLLUSCA. MACTRADtE. Mactra.449. M. cinerea. — Shell subtriangular ; beaks prominent,incurved, from which there is a depression to both extremities.Turt. Biv. Brit. 73.—Weymouth and Paing-Mont. Test. Brit. Sup. 36.ton Sands.This species bears nearly the same relation to the preceding, as M.truncata bears to M. solida. Its valves are more concave, more angulated,and rather broader in proportion to their length. It is nearly destitute ofmarkings.450. M. glauca.— Shell oval, flattish, inequilateral and striatetransversely, with the beaks incurved.Don. Brit. Shells, t. cxxv. Mont. Test. Brit, nlxxi. Turt. Biv. Brit.74 and 258.— Shores of Cornwall, rare.Length nearly 2|, breadth 3h inches ; colour pale, with faint irregularglaucous rays ; beaks recurved, not quite central, with a narrow gape underthem.451. yL.fragilis.—" Shell oval-oblong, flattish, nearly equilateraland smooth, with an angular flexure at the anteriorside."Turt. Biv. Brit. 74. t. iv. 10—Near f.Guernsey.Length 1|, breadth near 2 inches, pale yellowish white, transparent,smooth, except a few irregular obscure transverse ridges, angular at themore produced side by a rib which runs obliquely from the hinge to themargin beaks ; pointed, not quite central, with a depression under them, onthe shorter side." Dr Turton considers this as the true M. fragilis of Chemnitz.452. M. dealbata.— Shell transversely oval ;finely striatedconcentrically, inequilateral.Pult. Dorset. 31. Mont. Test. Brit. 95. t. v. f. 1. Maton and Backett,Linn. Trans, viii. 68. t. i. f. 10.— Weymouth, Mr Bryer.Length 1 Jth, breadth lfth inches, Urn, pellucid, white; margin plain,edge sharp. According to Montagu, the middle tooth in one valve " is broad,bifurcated, angulated close to the beak ; lateral teeth not very remote ;"while the authors of the Descriptive Catalogue quoted above, describe the hingethus," cardo valvulae unius constat ex foviola triangulari, cum dente complicate,antrorsum porrecto, et denticulo laterali, postice inclinato ; alteriusex foviolis duabus triangularibus." It is probable that a more minute examinationof the hinge in this and M. deaurata would lead to their insertionin the genus Erycina of Lamarck.EXTINCT SPECIES.1. M. arcuata.— Ovate, smooth, back and anterior margin arched ; shell ofan uniform thickness ; posterior side smallest ; hinge narrow ; lateral teethstriated.— Sower. Min. Conch, t. clx. f. 1. 6.—In Crag at Holywell.2. M. dubia— Ovato-triangular, transversely elongated, smooth, thickenedtowards the margin, sides equal.— Sower. Min. Conch, t. clx. f. 2, 3, 4 InCrag.3. M. ovalis.— Oval, equilateral, smooth, thickness uniform.—Sower. Min.Conch, t. clx. f. 5.—In Crag.