8* SHELLS AND SHELL-FISH. PART I. 



dinate variations, such groups are the hest calculated 

 for discovering the true principles of variation. The 

 interchange of characters between one division and 

 another is better marked — the gradations more gentle 

 — the links more perfect ; and hence, by carefully 

 tracing the transitions from one form to another, we have 

 a better chance of correcting our previous errors, and 

 of discovering the fallacy of any preconceived theory, 

 than if the links were so wide apart as to leave the 

 imagination to fill them up, by supposing that they have 

 existed, or that they are not yet discovered. The 

 complete analysis of any one group, where these defi- 

 ciencies do not occur, is far more worthy the attention 

 of the scientific, and even the general, reader, than 

 numberless others whose links are incomplete. 



(73.) The most striking characteristic of the shells 

 composing this sub-family, next to their pear-shaped 

 form, is that belonging to the inner lip and the colu- 

 mella, or pillar. It will be remembered, that in the Pur- 

 purinrB, the inner lip, properly speaking, is merely a 

 thin glazing spread over that part of the pillar which 

 forms the left-hand side of the aperture : in this respect 

 they differ not from the shells now before us ; but the 

 pillar, instead of being broad, flat, and even concave, as 

 in all the Purpura, is invariably rounded, and conse- 

 quently very convex : the shape of the Pyrul<s, also, 

 necessarily renders the base of the shell much narrower, 

 and the canal much longer, than in their prototypes ; 

 although the degree of length of this channel, as well as 

 that of the spire, cannot enter into the general defini- 

 tion, since it will be seen that these are subordinate cha- 

 racters. The five principal modifications of form under 

 which we arrange all the species, are these : — 1 . Pyrula, 

 where the shells are strong and solid ; the whorls ge- 

 nerally muricated ; the spire short, yet elevated, well 

 defined, and often slightly papillary ; the inner lip en- 

 tirely wanting; and the canal considerably lengthened. 

 The Pyrula perversa may be taken as the type of this 

 genus, all of which are strong, solid, and muricated spe- 



