286 Mr. Swainson's Monograph 



leones. Lister, 974. f. 29. Gualt. tab. 43. f. T. Knorr. 2 tab. 16. f. 4. 5 

 Martini, 4 tab. 122. f. 1117. Ency. Meth. pi- 401. f. a. b. Sow. Ge- 

 nera. b. c. 



This common shell has been so often described, either as a Bucci- 

 num or an Eburna, that a detailed account of it is here unnecessary. 

 Its size, shape, and proportions are those of A. Tankervillii and 

 rubiginosa, from which it differs in being umbilicated : the base 

 has two thick and nearly equal belts, the upper one margined by a 

 deep groove, wherein are two impressed lines, but it wants the ad- 

 ditional belt, which surmounts those lines, in halteata and nivea. 

 Like all the other ancillaricE, having a grooved base, there is a mu- 

 cronate tooth on the edge of the outer lip. Inhabits the Indian 

 Ocean. 



M. Lamarck, it is well known, has placed this shell among his 

 EburncE, probably on account of its being umbilicated ; for it ap- 

 pears that no writer is, as yet, acquainted with its inhabitant. The 

 propriety of this arrangement has been 'questioned by others, and 

 is, I think, proved erroneous, by the fact of the other Eburnce being 

 naturally covered by an epidermis ; a presumptive proof that their 

 animals are formed on a very different construction to that of 

 Ancillaria glabrata, which is a naturally polished shell, and conse- 

 quently either wholly, or in part, covered by the dilated mantle of 

 its inhabitant. Its relation to the Eburnce is therefore more that 

 of analogy than of affinity. 



On the other hand, setting aside the very obvious similarity of 

 Tiahit between A. glabrata, and the Lamarckian Ancillaria:, the 

 discovery of the new species here described, unites them so gra- 

 dually and so naturally, that no other character can be used to 

 separate them generically, than that some are imperforate and 

 others are not. The very little importance that belongs to the um- 

 bilicus is generally known ; for we see its presence or absence in 

 different growths of the same shell; it cannot, therefore, have 

 much to do with the structure of the animal. Still less can aa 

 umbilicus constitute a type of form ; or, in other words, a genus. 

 Admitting, therefore, that A, glabrata belongs not to the La- 



