GALLERY."! natural history. 119 
horse-shoe shaped muscular impression, like an operculum; 
and the latter forms a depression by corroding a space on 
the surface of the shell to which it is attached of the size 
of its own shell, and marked with a crescent-shaped ridge, 
shewing the place where the muscles were affixed : the ge¬ 
nus Brocckia only differs from Capulus in having a broad 
sinus on the right side of the aperture. 
The family of Slipper Shells ( CalyptrceidcE ) chiefly dif¬ 
fer from the former family in the body and shell being 
somewhat spiral, but they differ from most other spiral 
shells in the hinder lip being deeply concave, and fur¬ 
nished with a much raised edge, so as to inclose the whole of 
the foot of the animal when it is living attached to marine 
bodies like the Limpets, which they greatly resemble in 
their shape and manner of life. The shells of this family 
greatly differ in shape; indeed, so much so that it is diffi¬ 
cult to believe how they could be formed by the same kind 
of animals, yet, when a large series of them are compared, 
they so gradually pass one into the other that it is not easy 
to separate them even into genera. In Crepidula and 
Calyptrcea the apex is spiral; in some of the latter genus 
the axis is perforated. In some species, otherwise allied to 
Calyptrcea , the spire is so reduced in length, and the la¬ 
mina which separates the whorls of the body is so rudi¬ 
mentary, that nothing but the part which surrounds the 
much enlarged perforation of the axis is left; these modi¬ 
fications, when fully developed, have been called the Cup 
and Saucer Limpet ( Dispoiea ). The Cremoria , on the other 
hand, have only a folded plate on the apex of the cavity to 
support the back of the body, which is probably a modifica¬ 
tion of the plate usually found between the whorls of the 
body. These animals also have the faculty of secreting a 
cup-shaped shelly plate to support their foot, like the Hip- 
pony ces, but it is distinguished from the plate of the latter 
by being furnished with an oval central scar. 
The family of Carrier Shells ( Phoridce) must be placed 
here provisionally until their animal is better known, 
although it has a horny subangular operculum like the 
Zoophagous Ctenobranchiate Mollusca. Their shell is coni¬ 
cal, much like a Calyptrcea in shape, but they have the 
peculiarity of attaching, as the shell is enlarged in size, 
stones, fragments of other shells, coral, and other marine 
