Shell similar to that of the Siphonariidae, but the internal muscle-impression is horseshoe-shaped, that is, open anteriorly and not on the right. Gill absent.
The Trimusculidae (synonym Gadiniidae) is a small family of about eleven species, only one of which occurs in Southern Africa. The sole genus is Trimusculus Schmidt, 1818 (synonym Gadinia Gray, 1824). Like some of the Siphonariidae they are truly amphibious and evidently can breathe both in air and under water. Despite this, they display a distinctly specialized life-mode, being characteristic of the roofs of caves and overhangs in exposed cliffs and along the edges of reef platforms. Here they may form such densely packed colonies that their serrated shell margins may interlock like jigsaw-puzzles. Although they avoid both light and direct wave-action, they are dependent on brisk water movement for food. During feeding the shell is dropped slightly to permit water to flow under the mantle, the creature orientating itself with its tail-end to the current. A curtain of mucus is then secreted by mantle glands in front of the head; this billows out in the current, and functions as a net to trap microscopic plants. Once it is laden with food, the net is pushed under the shell by two prominent lobes that flank the mouth, and is then consumed.
As in the Siphonariidae the eggs are laid in gelatinous collars attached to the rock surface. However, in the Trimusculidae these are brooded beneath the mantle, either as a horseshoe-shaped mass or as two separate collars, one on either side of the foot. Each mass contains very numerous eggs, which hatch as crawling young, with a spiral shell and very large foot. Before settling and forming a permanent scar, each juvenile passes through an active crawling stage during which it grows rapidly.