158 INTRODUCTION TO CONCHOLOGY. 



the shell remarkable for its beautiful varieties of paiuting. 

 The second^ consisting of the genera Natica and Sigaretus, 

 is characterized by an animal of larger proportions, gene- 

 rally more or less enveloping the shell, by a thickened ridge, 

 and dilated extension of the foot. The species of this 

 division are uniformly marine. 



Such are the two separate groups into which the family 

 Kerifacea is divided ; and these, like nations, have their dis- 

 tinctive characters. 



The genus Kavicella comprises a small genus of fresh- 

 water mollusks, of which the shells are singularly depressed 

 and convoluted, resembling that of the Slipper Limpet, 

 although having little affinity with the Limpet tribe. They 

 inhabit flowing streams, and their symmetrically formed 

 shells are not subject to the distorted irregularities of 

 growth which pertain to such as live attached to the rough 

 surface of marine rocks. 



Their shells are generally mottled in lines or subtrian- 

 gidar patches, radiating from the apex, and they are covered 

 with a thin fibrous olive epidermis, the interior being mostly 

 of a bluish tinge. The operculum is composed of two parts, 

 doubtless with a reference to the habits or location of the 

 animal : the one is internal, imbedded between the middle 



