1 



extend quite, or more than half-way, to the bodj- 

 whorl, that the species should be regarded as plicate 

 or carinate. Whether species not usually j^Ucate do 

 not in some localities become so, from the absence of 

 disturbing influences of the waters, is a question 

 that we cannot as yet definitely decide ; its decision 

 in favor of such occasional development of plica? 

 would affect the validity of many species which are 

 now regarded as established. 



The development of carinse or tubercles on the 

 body-whorl of the adult shells is not nearly so 

 constant a character as would, at first sight, appear 

 to be the case, and several species are in doubt on 

 this account. Generally, however, these may be 

 regarded as more permanent characters when devel- 

 oped on the body than on the spire, as an adult 

 shell is not subject to the same mutations of form as 

 a juvenile individual. 



Of course, the relations of size and texture are 

 applicable to adults only ; and then the former is 

 subject to much variation from external influences. 

 Texture is an important, because a tolerably perma- 

 nent, discriminative guide. 



Color, external or internal, generally should not 

 be much relied on, nor the presence or absence of 

 bands, or maculations ; but in exceptional cases it 

 is very characteristic, as in P. viridulum, Anth., for 

 instance. Perhaps color in the interior is a more 

 reliable feature than epidermal or external hues. 



In some species, however, the presence or absence 

 of bands forms a prominent distinctive feature. 



Form, though subject to variation, may be relied 

 on as one of the best characteristics; the length, 

 number, and the convexity of the whorls, relative 

 size of the aperture to that of the entire shell, shape 

 of the outer lip and of the columella, are all gener- 

 ally reliable. 



To repeat ; in distinguishing a species of Strepo- 

 7natida, of course the first step is to ascertain 



