120 UNIVALVES. NERITA. 



There is considerable variation in llie form and mark- 

 ings of the Neritae: some are spiral, with prominent 

 whirls; others have their whirls partly or wholly con- 

 cealed ; some, again, are umbilicatcd, while others are 

 perfectly entire and solid; and many have the umbilicus 

 [)artially covered by a repand lip, or fissurated nodule. 



The interior of the mouth and lips is, in many species, 

 toothless; but in others, both lips are beset with strong 

 prominent, and articulated teeth, often terminating in dis- 

 jointed, elevated striae, or protuberant granulations. 



In most species, the back of the shell is covered with 

 strong, elevated ribs, sometimes nodulous and imbricated; 

 it is often only minutely striated, and has frequently a 

 perfectly smooth surface and brilliant polish. 



Of the umbilicatcd species may be particularized the 

 N.canrena, of which there are many beautiful varieties; 

 the N. cancellata, N. glaucina, N. vitellus, and" N. mam- 

 milla. The common variety of the latter shell is white, 

 having a porcelain appearance; but the rarer varieties 

 incline to a brownish orange, having their lips surmount- 

 ed with a black margin or border. The N. stercus mus- 

 carum is beautifully dotted with brown, or rufous, on a 

 clear white ground. The N. fulminea is marked with 

 angular stripes, resembling forked lightning. 



The next division of the Nerites consists of those which 

 are imperforated and toothless, as the N. corona and N. flu- 

 viatilis; the former is often of a blackish color, and has its 

 whirls crowned w ith spines of an unequal length : the lat- 

 ter is an inhabitant of the rivers of Europe and Barbary ; 

 it is usually marked w ith scaly spots, and is sometimes 

 rugged, streaked, or reticulated. 



The species of the third division are distiijguished from 

 those of the preceding by having Iheir lips toothed: the 



