Holotype: length 13 mm, width 5,9 mm; Largest paratype, length 13,4 mm, width 5.0 mm; Smallest paratype: length 9,4 mm, width 4,3 mm. [Radwin & D'Attilio, 1976]

Known only from Puerto Rico and St. Thomas, Virgin Islands.

The shell is of moderate size for the genus ( maximum length 13.4 mm) and lanceolate, The spire is high and markedly acute, and consists of one and one-third nuclear whorls and six or seven broad, flattened postnuclear whorls. The suture is impressed and obscured at intervals by narrow varical buttresses. The body whorl is of moderate size, broad, and flattened. The aperture is small and ovate, with a barely perceptible trace of an anal sulcus. The outer apertural lip is weakly erect and bears five very weak denticles on its inner surface. The columellar lip is smooth, detached, and erect. The siphonal canal is moderately long for the genus, moderately open, bent to the left, and moderately dorsally recurved.
The body whorl bears four moderately broad lateral varices, the right ventral and left dorsal ones most prominent. Two moderately strong costae, representing the single ventral and dorsal varices seen on the first two or three postnuclear whorls, are apparent on the body whorl. Spiral sculpture is present only in the intritacalx. Six broad cords with equal interspaces are apparent on the varices and become more apparent between the varices as the intritacalx is eroded. The microsculpture of the intritacalx consists of axial striae and numerous transverse tubes.
The shell is translucent white, covered by a flat-white intritacalx and a yellow-brown periostracum. The aperture is porcelaneous yellowwhite.
The radular dentition is muricine and is very similar to that of A. pollux. The rachidian tooth bears five cusps; the relative lengths of the central, lateral, and intermediate cusps are in the ratio of 5:3:2.
Radwin & D'Attilio, 1976. - Original Description.

This species resembles A. senex and A. pollux but differs from these species in several ways. It differs from A. senex in its relatively more slender shell, its more rounded (almost circular) aperture, its detached and erect columellar lip, its weaker denticulation of the inner surface of the outer apertural lip, and its more flattened body whorl. A. castor differs from A. pollux in its coarser axial intritacalx sculpture, its more slender and more gradually tapering spire, its weaker, unstained denticles en the inner surface of the outer apertural lip, the occurrence of two (rather than two and one-half ) buttresses between varices, and the fact that the varical flange on the body whorl meets the flange on the preceding whorl directly (rather than passing behind the earlier flange, as in A. pollux).
Radwin & D'Attilio, 1976.