Synopsis Frksh-Wate;r Mollusca 



Genus GYROTOMA Shuttle worth, 1845. 



37 



Shell of moderate size, spiral, dextral, imperforate, smooth 

 or spirally striate, rather thick and solid, conical or glohosely 

 ovate; aperture oval or elliptical, with a sutural slit or fissure 

 above, entire below ; lip thin and sharp. 



Fig. 133 Type: G. ovoidca Shutt., fig. 133. Radula, fig. 134. 



Genus ANCULOSA Say, 1821. 



Shell of moderate size, dextral, imperforate (except in one species), 

 smooth, tuberculate, spirally striate, sulcate or carinate, thick, solid, sub- 

 globose with a very short spire or thinner and conical ; aperture oval or sub- 

 circular, entire, rounded below ; columella callously thickened ; lip simple, 

 acute. 



Type: Melania prcrrosa Say, fig. 135. Radula, fig. 136. 



Order ASPIDOBRANCHIA. 



Suborder RHIPIDOGLOSSA. 



The Rhipidoglossse difiter radically from the Tasnioglossas in the charac- 

 ter of the lingual dentition. The radula has many rows of teeth, consisting 

 of a central, 2-5 laterals and numerous marginals arranged like the sticks 

 of a fan. 



Family NERITID^. 



Head large, rostrum divided and lobed in front; tentacles long and 

 slender ; eyes carried on peduncles placed at the external base of the tenta- 

 cles; foot large, truncate before and obtuse behind, sides simple; gill large, 

 triangular, pointed, free at its extremity; branchial and excretory orifices 



