EXPLANATION OF TERMS. 



353 



Crenated. Notched. 

 Crenulated. Full of notches. 

 Cuneiform. Wedge-shaped, as Donax. 

 Cuticle. The skin. 



Deciduous. Falling off. 



Decollated. Having several whorls broken off. 

 Deltidium. A three-sided space. 

 Dentated. Toothed. 

 Denticle. A small tooth. 



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 processes. 



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. 

 Elliptical. Oval. 



Emarginate. Notched at the edge. 

 Embryonic. Immature, unfinished. 

 Ensiform. Shaped like a sword. 



Entire. Having the aperture undivided at the base by a channel. 



Epidermis. The membranous covering of some shells. 



Epiphragma. A covering to the aperture of some spiral shells, which 



(unlike the operculum) is not attached to the animal. (See 



Operculum.) 



Equilateral. When the two sides of a Bivalve are equal, the bosses 

 being nearly or quite in the middle : as in the cockle. 



Equivalue. When the two valves of a Bivalve are equal in size ; as in 

 the cockle. 



Eroded. Gnawed, or rubbed. 



Fibi^ons. Resembling fibres : applied to the substance of a shell, it 

 indicates those, like the Pinna, whose fracture presents perpen- 

 dicular fibres. 



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