This spoon has a shell on both it's handle and on the bowl's back.  The handle is also monogrammed, horizontally, MAC in attractive script.

On the back it is stamped with Bust Lion D, according to John McGrew's "Manufacturers' Mark on American Coin Silver, is an unidentified New York City maker.  The H & B retail stamp is for either Hall and Brower Albany or Hutten & Bullinger NYC.  Circa 1840s/50s.