THE NATURAL HISTORY SURVEY. 59 
Shell: Of good size, quadrangular, compressed, thin; rounded 
before and squarely truncated behind; dorsal margin straight, 
ventral margin slightly rounded; surface roughened by the lines 
of growth, which are more or less sharp and elevated on the 
borders; umbones small, very much depressed, compressed, 
reddish brown, marked by about six well-developed, large, ele- 
vated, undulating ridges; anterior umbonal slope gently 
rounded, posterior slope rounded and slightly excavated; liga- 
ment short, wide, dark brown or black; epidermis reddish, cor- 
neous, yellowish or greenish, sometimes blackish, frequently 
covered with a deposit of foreign matter and rayed similar 
to A rugosa; cardinal teeth double in both valves; in the 
left valve there is an elevated, rounded tooth just below the 
apex of the umbo, then a smaller, long and narrow tooth 
in front of this; in the right valve there is a very large, 
long and narrow tooth anterior to the umbo, and a very small, 
long and narrow thickening of the hinge line above this; some- 
times, in very old specimens, there is a third prominent tooth 
between the two just mentioned in each valve; the cardinals are 
peculiar in being long, narrow, elevated and thin; they are 
smooth, or only very slightly roughened at their apices; lateral 
teeth weak, long, thin, lamelliform, smooth; there is frequently 
an indication of a second lateral in the right valve; anterior ad- 
ductor muscle scar longer than wide, impressed, smooth; pos- 
terior adductor muscle scar not very distinct, confluent, diam- 
eters equal; protractor pedis muscle scar wider than long; 
slightly impressed; dorsal muscle scars scarcely visible, situated 
on the posterior face of the cardinal tooth; pallial line distinct, 
crenulated, impressed anteriorly; cavity of the beaks very shal- 
low; nacre pearly white, very iridescent, inclining in some speci- 
mens to flesh color. 
- Length, 85.00; height, 50.00; breadth, 26.00 mill. (12811). 
us Gore i 39.00; e ASG) ee (12411). 
Animal: Generally dirty white, black on tentacular portion 
and edge of mantle; ctenidia large, rounded before and obtusely 
pointed behind, generally short and wide, attached above as in 
A. rugosa; labial palpi small, subtriangular, united at base and 
partly above; siphons dirty white inside, blackish on the edge; 
branchial opening large with papilla; cloacal opening small 
without papillae; foot thin, dirty white in color; mantle thin, 
somewhat thickened on the border. The whole outer bran- 
chium is occupied as an uterus. 
