56 PHYLOGENY OF FUSUS AND ITS ALLIES. 



protoconch, the bicarination of the young, which is not characteristic of 

 other species of Fiisus, except the closely related F. novcB-hollandice, 

 and the general form and character of the spindle. It is not improb- 

 able that F. longirostris is in the line of ancestry of F. dupetit-thouarsii 

 and its allies. This suggests that the migration of the ancestors of the 

 latter species occurred in Tertiary times, a supposition which needs for 

 confirmation the finding of Tertiary species related to F. dupetit- 

 thouarsii in American deposits. In this connection the following 

 species is suggestive, but not conclusive. 



FUSUS GABBI sp. nov. 



i860. Fusus henekeni Gabb, Journ. Acad. Nat. Sci. Phil, 2d sen, vol. 8, p. 350, 

 pi. 45, fig. 31. 



This species was originally described by W. M. Gabb, who referred 

 it to F. henekeni Sowerby (see p. 19). It is, however, very distinct 

 from that species, nor can its relation to F. dupetit-thouarsii be con- 

 sidered as proven. Gabb says it suggests "irresistibly" this species, 

 but in this I differ from him most decidedly, though I admit that the 

 general form and proportions are not unlike those of that species. The 

 exact relation of F. gabbi to other species has not been determined, 

 since the apex and early whorls of the only specimen known are in 

 part broken away and in part badly worn. It may be a descendant 

 of F. longirostris or it may be related to F. henekeni, but for neither of 

 these suppositions is there any good evidence. 



The youngest whorls observed are round, rather more embracing 

 than F. henekeni, and have round and rather distant ribs. When 

 still quite young the whorls become angulated at the periphery, this 

 angulation being due to the strengthening of the central spiral. The 

 shoulder becomes flattened and the ribs become obsolescent towards 

 the sutures. Simultaneous with the angulation intercalated spirals 

 make their appearance. On the final whorl, the ribs are reduced to 

 mere undulations, the lip showing senile characteristics. These are 

 accompanied by an irregular thickening of the lip and the formation 

 of strong lirse within. Strong plications occur on the columella, but 

 these appear to be due mainly to the influence of the columellar spirals, 

 which are not obliterated by resorption or covered by a thick inner lip. 



If this species could be shown to have ancestral relations to F. 

 dupetit-thouarsii it would be of great interest, as showing the way by 

 which the ancestors of that species reached the new world from their 

 place of origin in the old. Such relationship is, however, not indicated, 

 and we are at present left without any clue to the origin of F. dupetit- 

 thouarsii, the most important American species of the genus. 



Locality: Costa Rica (Phil. Acad. Sci. Gabb's type). 



H orison: Pliocene ? (Gabb). 



