392 MOLLUSCA. 
Mactra MARcIDA (Gould). 
T. ventricosa, ovato-triangularis, cretacea, epidermide tenui stramineo 
induta; umbonibus medianis, tumidis, remotis, viz anteversis ; latere 
antico breviori, rotundato; latere siphonali subtriangulato, acuto ; 
margine ventrali arcuato: fovea ligamentalis ampla; dente V-formi 
parvo, prominente ; dentibus lateralibus crassis, parum elevatis: wnte- 
rior cretacea; sinu siphonali parvo, trientem test@ attingente. 
Mactra marcida, Goutp; Proceed. Boston Soc. Nat. Hist., ii. 216. 
May 1850. Expedition Shells, 76. 
SHeE.L of medium size, solid, coarse, chalky white, covered with a 
very thin straw-coloured, shining epidermis ; surface irregularly undu- 
lated concentrically. Form rounded ovate-triangular, nearly equi- 
lateral; anterior end obtusely rounded, narrower than the posterior, and 
a very little shorter; posterior end short, triangular, obtusely rounded 
at point, very nearly closed. Beaks prominent, tumid, the points rather 
remote from each other; anterior edge direct for half its length, then 
arching downward, very broad, rounded, without distinct areola, a 
faint line on each valve enclosing a large space ; posterior edge broad, 
obtuse, regularly arched, not truncate; umbonal slope tumid; valves 
ventricose, rather thick, chalky white within; siphonal sinus small, 
traversing about one-third of the valve; cardinal area median ; cartilage 
pit deep; tooth strong, sides equal; lateral teeth strong, elevated, tri- 
angular. 
Length two and three-fourths inches ; altitude two and one-fourth 
inches; breadth an inch and a half. 
Inhabits Orange Harbour. 
This species has precisely the form of M. dactea, but is a much 
more solid and rude shell, wanting also the decided dorsal angularities 
of the umbones. 
Figures 505, 505 a, lateral and dorsal views of the shell ; 505 4, inte- 
rior, with the hinge. 
