11 



26. Neritina. Shell semiglobular, flattened under- 

 neath, without umbilicus ; the partition flattened, 

 rather acute, neither toothed nor crenulate : oper- 

 culum horny, with a spine. 

 Type. Nerita fluviatilis, Montagu. 



g. Patellidce. 



Animal inhabiting fresh water, enveloped in a conic 

 shell, which is concave underneath. 



27. Ancylus. Shell thin ; the crown imperforate, 

 and incHning posteriorly : spire none : margin en- 

 tire : muscular impression somewhat orbicular, and 

 emarginate anteriorly. 



Type. Patella fluviatihs, Montagu. 



