INSTITUTE OF SCIENCE, PHILADELPHIA. 173 



by the spirals ; lastly, by very fine, close, even, little-elevated ridges with equal 

 interspaces, covering the whole surface, but only visible where it is well pre- 

 served ; suture well marked; pillar stout, a little recurved; canal short; 

 aperture narrow, pointed at each end ; outer lip prominent, varicose, con- 

 stricted in the middle, inside thickened and contracted ; body with a thick, 

 elevated callus ; these two masses of callus approach and nearly meet behind, 

 being barely separated at the posterior commissure, behind which the gutter 

 widens somewhat with elevated sides, and is continued nearly to the apex of 

 the spire and recurved on the other side for a short distance. Max. Ion. of 

 shell 24.0; max. lat. lo.o mm. 



The specimen from Martin Station was a siliceous pseudomorph, the 

 Ocala specimen was a mold, both imperfect, but affording the above characters 

 by combination. There is no other American species resembling it at all 

 nearly, so that comparisons are unnecessary. 



Genus STROMBUS Unni. 



This genus, derived from Linneus, who gave it a name of very ancient 

 date, but which had not previously been restricted to a natural group, was re- 

 vised by Lamarck, whose sole example (Prodrome, lygg) was S. pt/gilis h., 

 which must be regarded as the type. The characters of the group in the 

 Lamarckian sense intergrade, so that it is difficult to divide the genus into sec- 

 tions. The typical species has an unguiculate operculum strengthened by a 

 midrib, and having its thin outer edge sharply serrate with angular retrorse 

 prickles. Nearly related species {S. inermis Swainson, 5. bitubercidaiiis 

 Lamarck) have both edges of the operculum entire and smooth, so that the 

 character used by Gabb to separate the Strombidce into subfamilies is not of 

 more than specific rank. The liration of the outer lip, which is present in 5. 

 pugilis of Eastern America, is absent in its west-coast analogue S. gracihor. 

 A remarkable character which I have not seen alluded to by anybody is the 

 development in many Strombs of elevated and complex plaits at the posterior 

 commissure of the aperture, though there are none on the anterior part of the 

 pillar. In accordance with the hypothesis broached in the earlier portion of 

 this paper, I regard this development as closely related to the posterior expan- 

 sion and attendant filament of the mantle in this genus, especially as it is more 

 marked in species which have a wide posterior expansion {S. galhis L., 5. 

 bituberculatus Lam., S. costatits Gmelin) than in closely related forms in which 

 the expansion is less (5. pitgilis, S. inermis Swainson, 5. integer Swainson), 

 whether these forms be regarded as distinct species or not. Furthermore, 

 these ridges are more numerous and more emphatic (though not always as 

 elevated) in the young shell with a narrow aperture, where the mantle when 

 withdrawn is much compressed, than they are in the same shell when adult, 

 when the aperture is expanded and the retracted mantle has more room. 



These plaits are best shown by sections of the adult shell exposing the in- 

 terior of the whorls. 



