10 NERITIDiE. 



at the apex ; aperture large, basal, with a small, not dentate col- 

 umellar shelf, and elongated lateral muscular scars. Operculum 

 much smaller than the aperture, irregularly subtetragonal ; labral 

 margin subcai'tilaginous ; coluniellar margin sinuous, partly bor- 

 dered by a parietal apophysis, projecting at its extremity ; apex 

 vermicularly granulated on the inner face, with a few radiating 

 strijB on both fiices (PI. 1, figs. 11, 12). 



Head large, eyes on long peduncles, foot large, regularly oval, 

 attached on each side to the visceral mass, forming a cavity open 

 behind, in which the operculum is j^artly buried. Dentition, PI. 1, 

 fig. 2. 



The species, about fifty in number, are exclusively East Indian 

 and Polynesian in distribution. They are usually found on the 

 banks of rivers adhering to floating sticks and to the petioles and 

 roots of the Nipah palms and other plants that live near the water ; 

 they are also found attached to smooth stones. 



The synonymy includes Septaria, Fer. 1807, which has priority, 

 but has not usually found accejitance, Catillus (Humphrey. 1797), 

 Swainson. 1840. 



Section Cimber, Montf 1810. 



Apex median, usually decorticated, projecting beyond the posterior 

 margin. N. porcellana, Linn. 



Dr. Gray has characterized three sections of this group, under 

 the names of Laodia, Elana, and Paria, all in 1867: they are dis- 

 tinguished partly from the opercula, partly from the shells. The 

 former are so variable in their characters that they aflford but 

 slender material for systematic purposes. The following may per- 

 haps be maintained : 



Subsection Paria, Gray. 1867. 



Septum projecting, and truncated in the middle. N. Freycineti, 

 Recluz. 



Section Stenopoma, Gray. 1867. 



Apex median, posterior, submarginal, entire; shell generally 

 narrow. N. lineata, Lam. 



Section Elara, H. and A. Adams. 1854. 



Apex a little elevated above the posterior margin and laterally 

 recurved. N. Lapeyrousei, Recluz. 



To these Gray adds a group Orthopoma, 1867, characterized by 

 an operculum only, the shell of which is unknown. 



