MOLLUSCA. 



207 



In the higher Prosobranchs the gills are comb-shaped 

 and the sexes are distinct. The group includes all the 

 spiral univalve sea-shells and a few fresh-water shells. 

 Many have the aperture entire, as the fresh -water 

 Paludina, the pyramidal Trochus, pearly Turbo, and 



FIG. 109. The Wentle-trap, (Scalana.) 



FIG. no. Volute Crawling. 



FIG. in. Murex. 



common Periwinkle (Littorind] from the sea. Others, 

 the highest of the race, have the margin of the aper- 

 ture notched or produced into a canal, and are carniv- 

 orous and marine ; such are nearly all the more beauti- 

 ful sea-shells, as the Cowry (Cyprced] Volute, (Fig. 1 10,) 



