Strombus costatus:

Ribbed or Milk Conch. A large and solid shell, which ranges in size from 10 to 18 cm long. About 10 whirls, producing a fairly tall spire. The body whirl is greatly expanded. A series of large blunt nodes occur on the last whirl, with a series of smaller ones twisting up the spire. There are also a number of rather distinct horizontal corrugations on the body whirl. The aperture is long and narrow, with an outer lip expanding, in older specimens this is quite thick. The colour is yellowish-white, and somewhat mottled; interior pure white, with a noticeable aluminum-like glaze on the columella and outer lip. This is the most abundant conch in Harrington sound. This is subtidal species.