What is "helmet shell"

Helmet \Hel"met\, n. [OF. helmet, a dim of helme, F. heaume; of Teutonic origin; cf. G. helm, akin to AS. & OS. helm, D. helm, helmet, Icel. hj[=a]lmr, Sw. hjelm, Dan. hielm, Goth. hilms; and prob. from the root of AS. helan to hide, to hele; cf. also Lith. szalmas, Russ. shleme, Skr. [,c]arman protection. [root]17. Cf. Hele, Hell, Helm a helmet.]

1. (Armor) A defensive covering for the head. See Casque, Headpiece, Morion, Sallet, and Illust. of Beaver.

2. (Her.) The representation of a helmet over shields or coats of arms, denoting gradations of rank by modifications of form.

3. A helmet-shaped hat, made of cork, felt, metal, or other suitable material, worn as part of the uniform of soldiers, firemen, etc., also worn in hot countries as a protection from the heat of the sun.

4. That which resembles a helmet in form, position, etc.; as:

   1. (Chem.) The upper part of a retort.
      --Boyle.

   2. (Bot.) The hood-formed upper sepal or petal of some flowers, as of the monkshood or the snapdragon.

   3. (Zo["o]l.) A naked shield or protuberance on the top or fore part of the head of a bird.

      Helmet beetle (Zo["o]l.), a leaf-eating beetle of the family Chrysomelid[ae], having a short, broad, and flattened body. Many species are known.

      Helmet shell (Zo["o]l.), one of many species of tropical marine univalve shells belonging to Cassis and allied genera. Many of them are large and handsome; several are used for cutting as cameos, and hence are called cameo shells. See King conch.

      Helmet shrike (Zo["o]l.), an African wood shrike of the genus Prionodon, having a large crest.