The Slipper Limpet (Crepidula fornicata), has a flattish shelf across opening which gives it the appearance of a slipper. It is quite a flattish shell. It may settle into stacks that curve into an arch shape due to their shape.
Slipper Limpet - WWC Archives
Description[]
The size of the shell is 20-50 mm. The maximum recorded shell length is 56 mm.
This sea snail has an arched, rounded shell. On the inside of the shell there is a white "deck" which causes the shell to resemble a boat or a slipper, hence the common names. Some of these may be flat, slightly arched, or arched heavily. The 'Flat slipper shell' is also another type of slipper shell.
Gallery[]
Slipper Limpet Stack - http://www.flickr.com/photos/qwertyqwertyqwerty/
Underside of Slipper Limpet - WWC Archives