ACEPHALA. 39] 
Mactra DELutTA (Gould). 
T. tumida, ovato-rotundata, hiantissima, sordida, epidermide tenu 
deluto, oblique corrugato induta, concentrice undulata; umbonibus 
submedianis, tumidis, incumbentibus, contiguis ; margine dorsal 
utroque dechvi ; margine ventrali rotundato; lateribus obtusis: car- 
dinis fovea ligamentali profunda ; dente V-formi gracili, ramis im- 
equalibus ; dentibus lateralibus brevibus, trigonis, compressis ; sinu 
siphonali propé ad centrum valve obducto. 
Mactra deluta, Goutp; Proceed. Boston Soc. Nat. Hist., i. 215. 
May 1850. Expedition Shells, 75. 
SHELL small, rather thin, tumid, rounded-ovate, nearly equilateral, 
dingy-white, with a thin, fugacious, obliquely wrinkled, dirty-yellow 
epidermis ; surface irregularly and delicately undulate concentrically ; 
beaks nearly entire, anterior end shortest, sloping rapidly, obtusely 
rounded; posterior end still more obtusely rounded, not truncate, 
widely gaping; ventral edge regularly arched ; anterior dorsal edge 
excavated near the beaks, without a distinctly marked areola, broad, 
striate ; posterior dorsal edge broad, rounded, excavated near the beaks ; 
beaks tumid, acute, contiguous, the two umbonal slopes about equally 
tumid. Valves globosely convex; hinge area large ; ligamentary pit 
deeply excavated ; V-tooth delicate, sides unequal in length and thick- 
ness; lateral teeth short, elevated, thin, triangular; siphonal sinus 
rather broad, extending nearly half across the shell. 
Length an inch and seven-eighths ; height an inch and six-eighths ; 
breadth an inch and one-tenth. 
Inhabits New Zealand. 
The external appearance is rather that of a rude Venus, and has 
fewer angularities than any other Mactra. It is also peculiar for its 
very wide gape. 
Figures 503, 503 a, lateral and dorsal views of the shell; 503 0, 
interior view, with the hinge. 
