Acorn barnacle and acorn shell are vernacular names for certain types of stalkless barnacles, generally excluding stalked or gooseneck barnacles. As adults they are typically cone-shaped, symmetrical, and attached to rocks or other fixed objects in the ocean. Members of the barnacle order Balanomorpha are often called acorn barnacles.[1][2]
In addition, several species of barnacles are specifically called by the common name "acorn barnacle", including:[3][4][5]
...in the Acorn Barnacle, Balanus,...
Suborder Balanomorpha (acorn barnacles); Balanus glandula (common acorn barnacle)