X INTRODUCTION. 



In regard to the admeasurement of shells in the present work, minute fractional 

 descriptions have not been very particularly attended to, but a general approximation 

 only has been given, from a belief that a comparison of the dimensions of the aperture and 

 the axis of a shell is a test of fallacious dependence. In many instances, specimens 

 of the same species have a much greater range in variation than, as a general character, 

 Avould be exhibited between two proximate species ; the aperture in some individuals 

 being equal to two thirds of the length of axis, while in others of the same species it 

 does not exceed one third. In an elevated or cylindrically formed shell, with numerous 

 volutions, the aperture of a young individual bears a greater proportion to the axis of 

 the shell than it does when the animal is fully grown ; and it is not always possible to 

 ascertain when an individual has reached a state of maturity. The form of the 

 aperture is nearly the same at all ages or states of the individual, but the lengthening 

 or shortening of the convoluted cone, which may be the result of external causes, will 

 materially affect any mathematical proportion that may be assumed as a standard for 

 specific determination ; and the spiral angle, or angle of volution,* upon which some 

 Continental conchologists have placed so great reliance, is a test, in my opinion, equally 

 illusory and deceptive. In Littorina littorea, for example, the variations extend, in 

 what I firmly believe to be individuals of the same species, from a subulate or tapering 

 form of volution, denoting an angle of scarcely twenty degrees, to another form which, 

 by the application of the same rule, would show an angle of ninety degrees ; this may 

 be seen in Plate X, where the figures are by no means exaggerated, but rather fall 

 short of the two extremes. A like want of permanent regularity in proportionate 

 dimensions is equally exhibited in the Acephala or bivalve molluscs ; and minute 

 attention to accurate detail or mathematical proportions is rarely to be depended 

 upon, even in that class. In some species, of which the general form may be con- 

 sidered as transverse, the character may be traced through trifling and almost imper- 

 ceptible gradations until it is decidedly elongate, so that mathematical proportions in 

 such cases are of no determinate value. This is more particularly evident in some of the 

 species in the Crag Formations, where, perhaps, a rather more than ordinary degree of 

 variation may be seen ; and such are the extraordinary varieties in many of the species, 

 that no reliance can be placed, for specific determination, upon minute proportionate 

 dimensions, although, as an auxiliary, they may in some cases be usefully refen'ed to. 



No alteration has been thought necessary in the generally accepted terms of 



* Tlie angle formed by a line drawn from tbe vertex on each side of the shell, which varies, of course, 

 upon the greater or less depression of the volution, the sutural angle, also, depending upon the same cause. 



