MOLLUSCA. MACTRADiE. 
Mactra. 
449 . M. cmerea. — Shell sub triangular ; 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. glaiLca. — Shell oval, flattish, inequilateral and striate 
transversely, with the beaks incurved. 
Dmi. Brit. Shells, t. cxxv. Mont Test. Brit. Dlxxi. Turt Biv. Brit. 
74 and 258. — Shores of Cornwall, rare. 
Length nearly 2|, breadth 3| inches ; colour pale, with faint irregular 
glaucous rays ; beaks recurved, not quite central, with a narrow gape under 
them. 
451. 'M.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, jmle 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. fragiHs of Chem- 
nitz. 
452. M. dealbata . — Shell transversely oval ; finely striated 
concentrically, inequilateral. 
Pult Dorset. 31. Mont Test. Brit. 95. t. v. f. 1. Matm and Rackett^ 
Linn. Trans, viii. 68. t. i. f. 10.— Weymouth, Mr Bryer. 
Length l|th, breadth Ifth inches, thin, 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- 
plicato, antrorsum porrecto, et denticulo lateral!, 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 Erydna of Lamarck. 
EXTIXCT 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. duUa — pvato-triangular, transversely elongated, smooth, thickened 
towards the margin, sides equal. — Sower. Min. Conch, t. clx. f. 2, 3, 4. — In 
Crag. 
3. M. Oval, equilateral, smooth, thickness uniform.— 5'own Min. 
Conch, t. clx. f. 5. — In Crag. , 
