136 



SYSTEMATIC ARRANGEMENT 



I. Riphidoglossa. Teeth transparent, glass-like, in very numerous 

 longitudinal series ; central 5 * 1 * 5, variable inform; lateral 

 very numerous, subsimilar, more slender, curved at the tip, the 

 inner ones on each side largest. Rectum traversing the heart. 



Suborder I. Scutibranchia. 



Gills in a spiral line on the left side of the gill-cavity. Body and 

 shell spiral. Eyes pedicelled, at the outer side of the base of the 

 tentacles. 



A. Sides of the foot without any membranaceous fringe, with 

 beards on its upper surface. Shell not pearly within. The 

 second and fifth central teeth on each side large. 



Fam. I. NERITINIDiE. 



Shell hemispherical, flat beneath ; spire lateral ; inner lip flattened, 

 transverse ; outer arched. Operculum with a process on the inner 

 side under the nucleus, forming a kind of hinge with the inner lip of 

 the shell. 



Tentacles elongate, subulate. Foot oblong. Teeth : the central 

 4 • 1 * 4, very unequal ; the outer teeth large, broad, dark ; the lateral 

 teeth very numerous, slender, hooked (fig. 70). 



a. Operculum horny, with a shelly coat on each side ; edge chan- 

 neled (fig. 71). 



1. Nerita. 



Shell thick, hemispherical ; inner lip acute, often toothed ; aper- 

 ture moderate or semicircular. 



Fig. 70. — Teeth of Nerita peloronta. 



