428 MOLLUSCA. MACTRADtE. Mactra. 



449. M. cinerea. — Shell subtriangular ; beaks prominent, 



incurved, from which there is a depression to both extremities. 



Mont. Test. Brit. Sup. 36. Turt. Biv. Brit. 73.— Weymouth and Paing- 

 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 of 

 markings. 



450. M. glauca. — Shell oval, flattish, inequilateral and striate 



transversely, 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 irregular 

 glaucous rays ; beaks recurved, not quite central, with a narrow gape under 

 them. 



451. yL.fragilis. — " Shell oval-oblong, flattish, nearly equi- 

 lateral and smooth, with an angular flexure at the anterior 

 side." 



Turt. Biv. Brit. 74. t. iv. f. 10 — Near Guernsey. 



Length 1|, breadth near 2 inches, pale yellowish white, transparent, 

 smooth, except a few irregular obscure transverse ridges, angular at the 

 more produced side by a rib which runs obliquely from the hinge to the 

 margin ; beaks pointed, not quite central, with a depression under them, on 

 the shorter side." Dr Turton considers this as the true M. fragilis of Chem- 

 nitz. 



452. M. dealbata. — Shell transversely oval ; finely striated 

 concentrically, 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 hinge 

 thus, " cardo valvulae unius constat ex foviola triangulari, cum dente com- 

 plicate, antrorsum porrecto, et denticulo laterali, postice inclinato ; alterius 

 ex foviolis duabus triangularibus." It is probable that a more minute exa- 

 mination of the hinge in this and M. deaurata would lead to their insertion 

 in the genus Erycina of Lamarck. 



EXTINCT SPECIES. 



1. M. arcuata. — Ovate, smooth, back and anterior margin arched ; shell of 

 an uniform thickness ; posterior side smallest ; hinge narrow ; lateral teeth 

 striated. — Sower. Min. Conch, t. clx. f. 1. 6.— In Crag at Holywell. 



2. M. dubia — Ovato-triangular, transversely elongated, smooth, thickened 



towards the margin, sides equal. — Sower. Min. Conch, t. clx. f. 2, 3, 4 In 



Crag. 



3. M. ovalis. — Oval, equilateral, smooth, thickness uniform.— Sower. Min. 

 Conch, t. clx. f. 5. — In Crag. 



