EXPLANATION OF TERMS. 395 



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many of the Helicidce, or land-shells, are of this 

 form. 

 Dorsal. All shells are dorsal^ because they are all 

 placed upon the back of the animal : the term^ there- 

 fore^ is only applicable to the valves of bivalves, and 

 merely serves to distinguish that part of their cir- 

 cumference on which the bosses are placed, in oppo- 

 sition to that which, when the animal is crawl- 

 ing, is nearest the belly. The upper part of a spiral 

 shell, when the mouth is downward, may be termed 

 its back, or dorsal surface. 



Emarginate. A small notch, or fissure, wherever it may 

 occur : thus the base of most predacous Testacea, not 

 provided with a canal, are emarginate ; so also is one 

 extremity of Parmophorus , although in a very slight 

 degree, while, in the genus Emarginnla, the notch 

 becomes a deep slit. 



Epidermis. A name generally applied to the outer 

 rough coating of shells, over which it spreads as a 

 fibrous horny skin, although not really such, being 

 destitute of sensation. 



Entire. Immarginate, or uninterrupted, in opposition to 

 emarginate. 



Equilateral. When both sides of a bivalve are equals 

 the umbones or bosses being nearly, or quite, in the 

 middle : this is nearly the case with the common 

 cockle. 



Equivalve. The two valves of equal size and depth, as 

 in the generality of bivalves. 



Fibrous. Resembling fibres ; applied to the substance 

 of a shell it indicates those, like the Pinna, whose 

 fracture presents perpendicular fibres. 



Fimbriated. Thin elevated processes, somewhat re- 

 sembling fins, possessed by many of the Murices, as 

 Murejc fimbriata, &c., and sometimes placed round 

 the aperture of cyclostomous land shells. 



