EXPLANATION OF TERMS. 



295 



spiral shells, round which the whorls convolute ; it is 



not present in Solarium, Delphinida, &c. 

 Concentric. Arranged in parallel curves or bands, having 



one common centre. 

 Convoluted. The whorls rolled regularly one over the 



other. 



Cordate. Heart-shaped. 



Corneous. Resembling the colour or substance of horn ; 

 the epidermis of some, and the operculum of other 

 spiral shells often present this appearance. 



Coronated. Crowned, towards the apex, by a row of 

 spines. 



Crenated. Notched. 



Cuneiform. Wedge-shaped, as Donax. 



Dentated. Toothed. 



Dextral. Spiral shells are so called when the mouth 

 faces the right hand of the observer, the shell being 

 held with the apex upwards. The great majority of 

 spiral shells are dextral, but frequently in the very 

 same species (as in Bulimus) individuals occur whose 

 aperture is reversed, or appears on the left side. 

 ( See Sinistral.^) 



Digitated. The expansion of any particular part, as in the 

 outer lip of the Pteroceras, into finger-shaped pro- 

 cesses. 



Discoid. Disk-shaped, much flattened. The fresh-water 

 genus Planorbis, and many of the Helicidce, or land- 

 shells, are of this form. 



Dorsal. Belonging to the back. 



Effuse. Having the lips separated by a gutter or channeL 

 Entire. Having the mouth undivided at the base by a 

 channel. 



