The Collaborative International Dictionary of English v.0.48:

Saddle \Sad"dle\, n. [OE. sadel, AS. sadol; akin to D. zadel, G.
   sattel, OHG. satal, satul, Icel. s["o][eth]ull, Dan. & Sw.
   sadel; cf. Russ. siedlo; all perh. ultimately from the root
   of E. sit.]
   1. A seat for a rider, -- usually made of leather, padded to
      span comfortably a horse's back, furnished with stirrups
      for the rider's feet to rest in, and fastened in place
      with a girth; also, a seat for the rider on a bicycle or
      tricycle.
      [1913 Webster]

   2. A padded part of a harness which is worn on a horse's
      back, being fastened in place with a girth. It serves
      various purposes, as to keep the breeching in place, carry
      guides for the reins, etc.
      [1913 Webster]

   3. A piece of meat containing a part of the backbone of an
      animal with the ribs on each side; as, a saddle of mutton,
      of venison, etc.
      [1913 Webster]

   4. (Naut.) A block of wood, usually fastened to some spar,
      and shaped to receive the end of another spar.
      [1913 Webster]

   5. (Mach.) A part, as a flange, which is hollowed out to fit
      upon a convex surface and serve as a means of attachment
      or support.
      [1913 Webster]

   6. (Zool.) The clitellus of an earthworm.
      [1913 Webster]

   7. (Arch.) The threshold of a door, when a separate piece
      from the floor or landing; -- so called because it spans
      and covers the joint between two floors.
      [1913 Webster]

   8. (Phys. Geog.) A ridge connected two higher elevations; a
      low point in the crest line of a ridge; a col.
      [Webster 1913 Suppl.]

   9. (Mining) A formation of gold-bearing quartz occurring
      along the crest of an anticlinal fold, esp. in Australia.
      [Webster 1913 Suppl.]

   Saddle bar (Arch.), one the small iron bars to which the
      lead panels of a glazed window are secured. --Oxf. Gloss.

   Saddle gall (Far.), a sore or gall upon a horse's back,
      made by the saddle.

   Saddle girth, a band passing round the body of a horse to
      hold the saddle in its place.

   saddle horse, a horse suitable or trained for riding with a
      saddle.

   Saddle joint, in sheet-metal roofing, a joint formed by
      bending up the edge of a sheet and folding it downward
      over the turned-up edge of the next sheet.

   Saddle roof, (Arch.), a roof having two gables and one
      ridge; -- said of such a roof when used in places where a
      different form is more common; as, a tower surmounted by a
      saddle roof. Called also saddleback roof.

   Saddle shell (Zool.), any thin plicated bivalve shell of
      the genera Placuna and Anomia; -- so called from its
      shape. Called also saddle oyster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

saddle oyster
    n 1: thin-shelled bivalve having the right valve deeply notched
         [syn: saddle oyster, Anomia ephippium]