Type locality: Massachusetts, "It occurs in the stomach of fishes, and has been found, after violent storms, on the shores of Massachusetts and farther north." (Say, 1826: 146). Description: Shell medium for the genus (usually between 70 and 99 mm, occasionally up to 116.1 mm), thick and solid. Shape variable. Spiral sculpture prominent, projecting, usually dark reddish brown. Spire whorls with 1-4 spiral cords, body whorl with 6-12 primary spiral cords. Interspaces smooth, occasionally with 1 fine secondary spiral cord. Range and habitat: Neptunea decemcostata is restricted to the northwest Atlantic and is known from New York in the south (S off Shinnecock Inlet) (pers. comm. & coll. Steve Rosenthal), along Massachusetts (USA) in the south to the Gulf of St. Lawrence (Canada) in the north. The species is well known along the coasts of Maine (USA) and Nova Scotia (Canada). Comparison: Neptunea decemcostata is variable in shape, number of spiral cords and colour but can be distinguished from all other Atlantic Neptunea species by the thick spiral cords with smooth interspaces. The colour ranges from white to pale brown, which is a well contrasting background for the brown or dark red brown spiral cords. Occasionally the shell is uniformly pale brown. Snow white or pale violet shells, known from off Nova Scotia, are rare. Neptunea lyrata from the north Pacific is similar in spiral sculpture and colour but differs in having a larger number of spiral cords on the spire whorls. A few specimens of Neptunea despecta known from Canada and described as Neptunea decemcostata clenchi in the same paper as turnerae, differ in the spiral sculpture which is closer to Neptunea despecta (more spiral cords, finer primary cords, the interspaces filled with fine secondary spiral cords of which the middle one is slightly stronger).

Neptunea decemcostata:
* Animal frequently pure white, sometimes flecked with blackish.
* Shell with posterior part of whorl generally tabulate or concave.
* Ribs strong, in adults broad and flattened on top, interspaces usually channelled, lightly grooved or smooth.
* General shape of shell less fusiform, and whorls less rounded.
* Aperture white; channels corresponding to the ribs, sometimes chestnut brown.
* Siphonal fasciole broad, excavated, and with the fold strong.
* Canal curved backward from the plane of the aperture.
* Outer margin of the mouth darker than the throat.

Neptunea lyrata
* Animal always black spotted ; end of siphon and proboscis black.
* Shell with posterior part of whorl rounded, making a smaller angle with the suture ;
* often with more or less strong ribs above the first prominent rib.
* Ribs slender, more equal, less elevated, not flattened on top ; interspaces not channelled, strongly grooved, often, with with quite prominent intercalated ribs.
* Aperture purple or livid; if partly white, the white is in the channels corresponding to the ribs, with the interspaces purple.
* Siphonal fasciole long and slender, usually nearly obsolete, sometimes quite so.
* Canal curved more or less strongly to the left of the aperture.
* Outer margin lighter than the throat.