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16.
([[underline]] Crucibulum spinosum. [[/underline]] No. 48a) has within it a thin, cup-shaped wall of shell, which in life is concealed within the body of the animal. The Slippers or Boat Shells ([[underline]] Crepidula fornicata [[/underline]] [[crossed-out]] No [[/crossed-out]]) often grow one upon another as shown in [[strikethrough]] the [[/strikethrough]] specimen [[strikethrough]] exhibited [[/strikethrough]], No. 48
Family Xenophoridae. No. 50, The Carrier Shells. 
In some manner not fully understood these animals cement pebbles, pieces of shell and coral to the outside of their shells, so that as they lie upon the bottom of