lO CONK LIN. [Vol. XIII. 



to the character of the surfaces upon which they are attached. 

 Upon a smooth, plane surface the shell is regular and unusually 

 broad and fiat ; on a convex surface it is deep and highly arched ; 

 on a concave surface it is concave, sometimes to the extent of 

 being almost semicircular ; on a twisted surface, like the colu- 

 mella of Neverita, it is twisted ; on an irregular surface, such 

 as a rough stone, it is irregular ; if pressed upon from the sides 

 the animal and shell become long and narrow; if growth is 

 limited in front the shell becomes short and broad ; if limited 

 on all sides the shell may increase greatly in thickness but re- 

 mains small, filling the space in which it is found. In such 

 cases the lines of growth are crowded closely together and the 

 very edge of the shell may be as thick as any other portion. 

 In small places, such as the interior of Illyonassa shells, C. plana 

 may be dwarfed to one twenty-fifth the size of normal specimens. 

 These individual variations in the shape and size of the animals 

 and shells appear in all the species of Crepidula, but they are 

 most marked in C. plana. The cause of the variations in the 

 shape of the shells is not far to seek, though the great differ- 

 ences in the size of individuals is more difficult to understand : 

 the shape of the shell is conditioned by the shape and position 

 of the mantle edge ; the mantle is moulded over the surface 

 upon which the animal rests ; and consequently the shape of 

 the shell comes in time to correspond to any sort of a surface 

 upon which the animal is attached. 



C. fornicata, the "slipper limpet" or "boat shell," is a com- 

 mon object to all visitors at the sea-shore. It occurs in great 

 numbers on the shells of the king crab, Limulus polyphemus, 

 where it is firmly attached to the ventral side of the carapace 

 and abdomen ; sometimes it is found on the appendages, the 

 gill plates, or even the dorsal surface of the crab. After it has 

 reached a certain size, about half that of the adult, it never 

 moves about. It thenceforth leads a perfectly passive existence, 

 being carried about by the king crab, and obtaining all its food 

 by merely sweeping into its mantle chamber currents of water 

 containing particles of food, which are in large part the crumbs 

 which have fallen from the king crab's table. This species is 

 also found abundantly on muddy sea bottoms a short distance 



