What is "snail trefoil"

Snail \Snail\ (sn[=a]l), n. [OE. snaile, AS. sn[ae]gel, snegel, sn[ae]gl; akin to G. schnecke, OHG. snecko, Dan. snegl, Icel. snigill.]

1. (Zo["o]l.)

   1. Any one of numerous species of terrestrial air-breathing gastropods belonging to the genus Helix and many allied genera of the family Helicid[ae]. They are abundant in nearly all parts of the world except the arctic regions, and feed almost entirely on vegetation; a land snail.

   2. Any gastropod having a general resemblance to the true snails, including fresh-water and marine species. See Pond snail, under Pond, and Sea snail.

2. Hence, a drone; a slow-moving person or thing.

3. (Mech.) A spiral cam, or a flat piece of metal of spirally curved outline, used for giving motion to, or changing the position of, another part, as the hammer tail of a striking clock.

4. A tortoise; in ancient warfare, a movable roof or shed to protect besiegers; a testudo. [Obs.]

   They had also all manner of gynes [engines] . . . that needful is [in] taking or sieging of castle or of city, as snails, that was naught else but hollow pavises and targets, under the which men, when they fought, were heled [protected], . . . as the snail is in his house; therefore they cleped them snails.
   --Vegetius (Trans.).

5. (Bot.) The pod of the sanil clover.

   Ear snail, Edible snail, Pond snail, etc. See under Ear, Edible, etc.

   Snail borer (Zo["o]l.), a boring univalve mollusk; a drill.

   Snail clover (Bot.), a cloverlike plant ( Medicago scuttellata, also, M. Helix); -- so named from its pods, which resemble the shells of snails; -- called also snail trefoil, snail medic, and beehive.

   Snail flower (Bot.), a leguminous plant ( Phaseolus Caracalla) having the keel of the carolla spirally coiled like a snail shell.

   Snail shell (Zo["o]l.), the shell of snail.

   Snail trefoil. (Bot.) See Snail clover, above.