The marsh periwinkle, Littorina irrorata, is an abundant snail in the salt marshes of the western Atlantic. The shell is elongate conic in shape, longer than it is wide (Andrews 1994). Coloration of the shell is dull grayish white with tiny dashes of reddish brown on the ridges of the spiral. Eight to ten gradually increasing flat whorls comprise the shell, with the body whorl measuring about half of the total height. The aperture is oval with a sharp outer tip and regular grooves on the inside edge.