The two species commonly known as horse clams inhabit the Pacific coast intertidal zones: the pacific gaper, "T. nuttallii", more abundant south to California; and the fat gaper, "T. capax", more abundant north to Alaska. Both have oval and chalky-white or yellow shells with patches of brown periostracum (leather-like skin) on the shell. These clams are also commonly called gapers because their shells are flared around the siphon and do not completely close, rather like geoduck clams. Like geoducks, they are unable to completely retract the siphon within the shell, though less flagrantly as the siphon on "Tresus" species is not as large.