152 SHELLS AND SHELL-FISH. PART I. 



inflected : hence these shells, on a cursory glance, much 

 more resemble the Buccininte than the Columbellince : 

 they are almost always smooth, and generally shining. 

 The 5th and last genus is Crassispira. (fig. 17. d.) 

 These are also small shells, having the outerlip, externally, 

 almost as thick as in Pusiostoma, but with a distinct 

 notch at the top ; this notch is in the same position, but 

 not near so deep, as in the club-shaped Pleurotomince : 

 like them, also, the spire is rather long, and the base 

 very short ; so that at first sight they remind us of very 

 small Cerithince : the aperture is contracted both above 

 and below, but wide in the middle ; thus being precisely 

 the reverse to that of Pusiostoma, which is contracted in 

 the middle, and wider at the extremities. These shells, 

 of which there are several species, are known at once 

 from the Pleurotomince, to which, however, they lead, 

 by the outer lip being inflexed and much thickened, 

 and by the sinus not extending beyond the margin. In 

 such species as unite with Pusiostoma, the aperture is 

 slightly toothed, and much contracted ; while in those, 

 on the other hand, which pass into JVitidella, the shell 

 is more fusiform, the mouth more effuse, and the 

 outer lip thin. These variations, which would otherwise 

 be unaccountable, are fully explained by the relations 

 that this singular little genus bears to the two adjoining 

 groups of the Columbellina, and render it one of the 

 most natural in the whole family. 



Analogies of the GENERA of the COLUMBELLINJE. 



Genera Typical Genera 



of the Analogies. of the 



ColunibellincB. Strombidte. 



Columbella. Sub-typical ; shell coniform. CONHS. 







C The outer lip often forming a si-T 



Nitidella. < nus at the upper part ; aper- > PLEUROTOMA. 



(. ture effuse. 3 



f Aperture very narrow; whorls } MAP 

 i thickened at their suture. 5 



This table is drawn up for the especial purpose of 



