394 SHELLS AND SHELL-FISH. 



bivalveSj which run parallel to the margin^ are termed 

 concentric. Among spiral shells these indications are 

 less common^ but the Wentletrap is a good example. 



Corneous. Resembling the colour or substance of horn : 

 the epidermis of some^ and the operculum of other 

 spiral shells^ often present this appearance. 



Coronated, or crowned. Spiral shells which have their 

 whorls more or less surmounted by a row of spines^ 

 or tubercles^ are termed coronated. The typical Vo- 

 lutes, or melon shells, several cones^ mitres, &c. are 

 thus ornamented. 



Crenated. Small indentations, generally sharp and re- 

 gular, often placed on the outer lip of spiral shells, 

 particularly on many of the typical mitres, as M. pa- 

 palis, episcopalis, &c. Harpa crenata is also a good 

 example : in bivalves fewer examples occur, and these 

 only in the teeth, as Byssoarca and Iridina. 



Cuniform. Wedge-shaped, as Donax : this shape, 

 comng bivalves, is analogous to that of clavate uni- 

 valves. 



Deciduous. Any part which falls off in a more ad- 

 vanced stage of growth, as the terminal whorls of the 

 pupaform land-shells, to which, indeed, this pecu- 

 liarity is almost confined. 



Dextral. When the mouth or aperture of a spiral 

 shell is on the right hand ; when on the left, it is 

 sinistral or reversed. The great majority of spiral 

 shells are dextral, but frequently in the very same 

 species (as in Bulimus), individuals occur whose aper- 

 ture is reversed. This, by the way, shows the in- 

 sufficiency of making such deviations the ground of 

 generic distinctions. In some genera, on the other 

 hand, as Physa, all the species are reversed. 



Digitated. The expansion of any particular part, as in 

 the outer lip of the scorpion Stromhi, into finger- 

 shaped processes. 



Discoid, or discoidal. Circular or disk-shaped, and 

 much flattened. The freshwater genus Planorbis, and 



