230 PYRAMIDELLIDiE. 



difficulty must ever exist iu ascertaining- the exact limits 

 of t'acli species, and indicating them by language. 



The present shell, by far the largest of our British 

 species, is lanceolate, with a moderately broad base, the 

 average length being to the breadth as three to one, or 

 seven to two. It is never much arcuated, thouorh often 

 slightly so in its upper volutions and earlier stages of 

 growth. It is strong, perfectly smooth, highly polished, 

 not compressed, and of an uniform ivory or cream white, 

 that often displays, from the semi-transparency of the 

 earlier turns — the lower whorls are opaque from their 

 superior solidity — the various hues of the animal inha- 

 bitant. The spire, whose sutural line is perceptible enough 

 for the genus, and a little slanting, consists of about eleven 

 whorls, that are not absolutely flat, though very nearly so, 

 are of slow longitudinal increase, of moderate enlarge- 

 ment in breadth (in the earlier turns, for the body and 

 penult are almost subcylindrical) and moderately short, 

 the dorsal breadth of the penult being just twice its length. 

 The body is half as long again as the preceding turn, and 

 occupies about two-sevenths of the total length ; its basal 

 declination in the adult is very gradual and moderately 

 rounded. The mouth, which scarcely fills one-fourth of 

 the entire length, is acuminated above, but is otherwise 

 oval, and not attenuated, but moderately rounded below. 

 The outer lip is rather thickened, and is either straightish 

 or very slightly sinuous above ; it arches and advances 

 below, where it pouts out more or less towards the base. 



The inner lip is rather broadly reflected over the pillar, 

 Avhich latter occupies one half of the length of the mouth, 

 and forms an angle with the base of the last turn. Our 

 largest example measured thirteen lines long, and was 

 three lines and a quarter in breadth. 



