Shell large, oblong, depressed, rather thin. Apex low, curving forward ; length of front slope contained about 3-1 times in the length of the shell; posterior slope gently convex. Surface covered with close, slightly unequal radiating riblets, each rendered rasplike by very close, regular and erect delicate lamellae; interstices narrow, having growth-striae but no lamellae.
The color is chestnut-brown, becoming dark umber in places, having short streaks and spots of white, forming a sparse tessellation. Inside light blue, with a small brown spot at the cavity of apex, and showing the colorpattern of the outside faintly through. Border wide, deep brown with white spots.
Length 46, breadth 32, alt. 61 mm.

The oblong, somewhat parallel-sided and depressed contour, thin texture, and the beautifully sharp and regular file-like sculpture of the low, close riblets, are its prominent features. It is allied to A. scabra, but the enormous number of specimens of that species which I have exam-ined in the Philadelphia and Washington collections, furnish no forms leading toward the dalliana.

Mexico. Angel Island, Porto Refugio, Gulf of California.