MOLLUSCS. 



345 



Ameru-ins call them idol sliells. A few sjjecies are fouiul in the southern portion of 

 North America, living in the rice swamps of Georgia, etc. 



The family Valvatid.e embraces a few small fresh-water shells, usually associated 

 with the PaludinidiX'. The shells are discoidal or conical and are usually covered witli a 

 greenish epidermis. The animal has a small foot, and when in motion the delicate bran- 

 chial plume is extended outside the gill cavity, and may be seen, an object of beauty, 

 above the neck. TJie species ai-e distributed through the temperate _JF^, 



regions of the globe, frequenting slow-running rivers and ponds. They ^^ 



are hermaphroditic and lay their eggs in a single capsule attached to fig.443. — Cai- 

 stones or to aquatic plants. Valvata, the only genus, has been divided 

 into three or four sub-genera. Our figure re]iresents Valvata tricarinata, a common 

 species in some parts of the United States. Other forms have the whorls rounded 

 instead of angular. 



We have already referred to some of the limpets on a preceding page, but it is ex- 

 tremely doubtful if all shoiUd be included in the Zygobranchiate Mollusca and if any 

 value is to be placed on lingual dentition, some of them should be assigned a position 

 here. We have already mentioned the Acmjsid^, and would merely state that usually 

 they are considered as closely allied to the Calyptk.eid^ which 

 will receive brief mention. In this family the shell is limpet-like 

 and with the beginning somewhat spiral. In some the spiral is so 

 far developed as to form a partial partition (columellar lip) so that 

 the common name 'slipper-limpet' is very ajijiropriate. In others 

 the interior of the shell is simple. Unlike the other limpets jire- 

 \iously mentioned, these forms are sedentary, and most of them 

 never seem to leave the spot where they settle down in early life. 

 „ . , , That this stationary habit occurs is shown by the shape of the 



Fig. 444. — Crepidula •' J i 



fnmicata, slipper shell, whicli is always moulded to fit any irregularities of the sur- 



limpet. . J t' 



face to which they are attached. Thus, when on an oyster shell, 

 the edge of the slipper-lim]iet reproduces (of course reversed) all the elevations and 

 depressions of the other. With this stationary life it is not readily seen what they 

 live upon, tuiless it be the microsco])ic life so abundant in the sea water, for they can- 

 not nu)\c far to ol)tain the attached alga;. In confinement some have lieen known 

 to devour animal food. 



The genus Crepidula departs but little in conchologieal characters from the ty2ie 

 usual in the univalve molluscs. The general ajipearance may be seen 

 from our figures, which rejiresent two of the common American forms. 

 The a|ierture is divided by a posterior lamina (the columellar lip) so 

 tliat the resemblance to a slipper is striking, whence the 

 common name, slipper-lim]>et. These forms are almost 

 always found .attached to the shells of other molluscs, 

 the outer part of the aperture of Natica and JVassa, when 

 inhabited by hermit crabs, being a favorite place. AVhy 

 this association of forms should e.xist has not been e.x- 

 pl.ained ; jiossibly the crumbs dropped from the hermit's dinner m.ay 

 afford abundant food for the limpet. Frequently several individuals are 

 placed one above another, the lowest one adhering to some living or dead 

 shell or to the king-ci-ab {Liinidus). 



The genera Calyptrcea and C'rucibnliiin are known as cup-and-saucer limpets. In 



