274 Mr. Swainson's Monograph 



others abound in a fossil state, in the London clay ; at Grignon, 

 Courtagnon, and the environs of Paris*. The genus is founded 

 alone upon the external characters of the shell ; no knowledge 

 having yet been acquired of its animal inhabitant. Yet from the 

 close resemblance between the shells of the AncillaricB and the 

 Olivce, we have every reason to suppose they will follow each other 

 in natural affinity, (along with the Volutce and other kindred 

 groups,) among the Gasteropoda PectinibrancJii oiM.Cuvier. The 

 natural situation of all these shells, however, is very uncertain, 

 and must be determined by the comparative anatomist rather than 

 by the conchologist, who should labour to render his study easy 

 and useful to the more important views of the geologist, and not 

 burthen it by long details on the structure of animals which it may 

 never be in the power of the student to see. I shall make some 

 further observations on this subject in another place. 



The shells of the AncillaricB are covered by a finely-polished 

 coating of enamel ; from which circumstance we have every reason 

 to conclude that the mantle of the animal (as in Cyprcea, some 

 Volutct, and other naturally-polished shells) is so much dilated, 

 as to fold over the sides, or even envelop the shell ; wherever this 

 is the case it generally happens that the animal is destitute of an 

 operculum, and the shell of any epidermis ; both of which, in fact, 

 would be useless. This remark I should v/ish the reader to bear 

 in mind, as I shall have occasion to revert to it hereafter. 



Ancillaria is known from Oliva by the suture being either quite 

 concealed, or (as in A. marginata) but slightly covered by a coat of 

 enamel ; whereas^ in Oliva, it is open, and remarkably channelled. 

 The thickened and striated base of the pillar is another, but not an 

 exclusive distinction ; as a similar structure will be found in many 

 of the African and South American Olives. 



The comparative length of the spire, as in most other genera, is 

 subject to some variation in the species, though to very little in the 

 individuals. I have therefore made use of this as a sectional cha- 



* BowDicH, Conch. Part I. p. 38. 



