What is "hedgehog shell"

Hedgehog \Hedge"hog`\, n.

1. (Zo["o]l.) A small European insectivore ( Erinaceus Europ[ae]us), and other allied species of Asia and Africa, having the hair on the upper part of its body mixed with prickles or spines. It is able to roll itself into a ball so as to present the spines outwardly in every direction. It is nocturnal in its habits, feeding chiefly upon insects.

2. (Zo["o]l.) The Canadian porcupine.[U.S]

3. (Bot.) A species of Medicago ( Medicago intertexta), the pods of which are armed with short spines; -- popularly so called.
   --Loudon.

4. A form of dredging machine.
   --Knight.

5. (Elec.) A variety of transformer with open magnetic circuit, the ends of the iron wire core being turned outward and presenting a bristling appearance, whence the name.

6. (Mil.) a defensive obstacle having pointed barbs extending outward, such as one composed of crossed logs with barbed wire wound around them, or a tangle of steel beams embedded in concrete used to impede or damage landing craft on a beach; also, a position well-fortified with such defensive obstacles.

   Hedgehog caterpillar (Zo["o]l.), the hairy larv[ae] of several species of bombycid moths, as of the Isabella moth. It curls up like a hedgehog when disturbed. See Woolly bear, and Isabella moth.

   Hedgehog fish (Zo["o]l.), any spinose plectognath fish, esp. of the genus Diodon; the porcupine fish.

   Hedgehog grass (Bot.), a grass with spiny involucres, growing on sandy shores; burgrass ( Cenchrus tribuloides).

   Hedgehog rat (Zo["o]l.), one of several West Indian rodents, allied to the porcupines, but with ratlike tails, and few quills, or only stiff bristles. The hedgehog rats belong to Capromys, Plagiodon, and allied genera.

   Hedgehog shell (Zo["o]l.), any spinose, marine, univalve shell of the genus Murex.

   Hedgehog thistle (Bot.), a plant of the Cactus family, globular in form, and covered with spines ( Echinocactus).

   Sea hedgehog. See Diodon.