The Collaborative International Dictionary of English v.0.48:

Soft-shell \Soft"-shell`\, Soft-shelled \Soft"-shelled`\, a.
   Having a soft or fragile shell.
   [1913 Webster]

   Soft-shell clam (Zool.), the long clam. See Mya.

   Soft-shelled crab. (Zool.) See the Note under Crab, 1.

   Soft-shelled turtle. (Zool.) Same as Soft tortoise, under
      Soft.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Long \Long\, a. [Compar. Longer; superl. Longest.] [AS.
   long, lang; akin to OS, OFries., D., & G. lang, Icel. langr,
   Sw. l[*a]ng, Dan. lang, Goth. laggs, L. longus. [root]125.
   Cf. Length, Ling a fish, Linger, Lunge, Purloin.]
   1. Drawn out in a line, or in the direction of length;
      protracted; extended; as, a long line; -- opposed to
      short, and distinguished from broad or wide.
      [1913 Webster]

   2. Drawn out or extended in time; continued through a
      considerable tine, or to a great length; as, a long series
      of events; a long debate; a long drama; a long history; a
      long book.
      [1913 Webster]

   3. Slow in passing; causing weariness by length or duration;
      lingering; as, long hours of watching.
      [1913 Webster]

   4. Occurring or coming after an extended interval; distant in
      time; far away.
      [1913 Webster]

            The we may us reserve both fresh and strong
            Against the tournament, which is not long.
                                                  --Spenser.
      [1913 Webster]

   5. Having a length of the specified measure; of a specified
      length; as, a span long; a yard long; a mile long, that
      is, extended to the measure of a mile, etc.
      [1913 Webster]

   6. Far-reaching; extensive. " Long views." --Burke.
      [1913 Webster]

   7. (Phonetics) Prolonged, or relatively more prolonged, in
      utterance; -- said of vowels and syllables. See Short,
      a., 13, and Guide to Pronunciation, [sect][sect] 22, 30.
      [1913 Webster]

   8. (Finance & Com.) Having a supply of stocks or goods;
      prepared for, or depending for a profit upon, advance in
      prices; as, long of cotton. Hence, the phrases: to be, or
      go, long of the market, to be on the long side of the
      market, to hold products or securities for a rise in
      price, esp. when bought on a margin. Contrasted to
      short.
      [Webster 1913 Suppl.]

   Note: Long is used as a prefix in a large number of compound
         adjectives which are mostly of obvious meaning; as,
         long-armed, long-beaked, long-haired, long-horned,
         long-necked, long-sleeved, long-tailed, long- worded,
         etc.
         [1913 Webster]

   In the long run, in the whole course of things taken
      together; in the ultimate result; eventually.

   Long clam (Zool.), the common clam (Mya arenaria) of the
      Northern United States and Canada; -- called also
      soft-shell clam and long-neck clam. See Mya.

   Long cloth, a kind of cotton cloth of superior quality.

   Long clothes, clothes worn by a young infant, extending
      below the feet.

   Long division. (Math.) See Division.

   Long dozen, one more than a dozen; thirteen.

   Long home, the grave.

   Long measure, Long meter. See under Measure, Meter.


   Long Parliament (Eng. Hist.), the Parliament which
      assembled Nov. 3, 1640, and was dissolved by Cromwell,
      April 20, 1653.

   Long price, the full retail price.

   Long purple (Bot.), a plant with purple flowers, supposed
      to be the Orchis mascula. --Dr. Prior.

   Long suit
      (a) (Whist), a suit of which one holds originally more
          than three cards. --R. A. Proctor.
      (b) One's most important resource or source of strength;
          as, as an entertainer, her voice was her long suit.

   Long tom.
      (a) A pivot gun of great length and range, on the dock of
          a vessel.
      (b) A long trough for washing auriferous earth. [Western
          U.S.]
      (c) (Zool.) The long-tailed titmouse.

   Long wall (Coal Mining), a working in which the whole seam
      is removed and the roof allowed to fall in, as the work
      progresses, except where passages are needed.

   Of long, a long time. [Obs.] --Fairfax.

   To be long of the market, or To go long of the market,
   To be on the long side of the market, etc. (Stock
      Exchange), to hold stock for a rise in price, or to have a
      contract under which one can demand stock on or before a
      certain day at a stipulated price; -- opposed to short
      in such phrases as, to be short of stock, to sell short,
      etc. [Cant] See Short.

   To have a long head, to have a farseeing or sagacious mind.
      [1913 Webster]

WordNet (r) 3.0 (2006):

soft-shell clam
    n 1: a clam that is usually steamed in the shell [syn: soft-
         shell clam, steamer, steamer clam, long-neck clam]
    2: an edible clam with thin oval-shaped shell found in coastal
       regions of the United States and Europe [syn: soft-shell
       clam, steamer, steamer clam, long-neck clam, Mya
       arenaria]