NERITINA. 53 



mantle. They have not been observed to floaty or creep 

 on the under surface of the water, which may account 

 for the shells being so often found encrusted with cal- 

 careous matter. Their tentacles, however, appear to be 

 extremely sensitive and always in motion. They are 

 vegetable feeders. Their eggs are generally deposited 

 and carried on the shell until they are hatched or de- 

 veloped. These are rounded, of a yellow colour, and 

 provided with a thick and leathery covering, which splits 

 in two when the fry are excluded, the upper half being 

 detached and the other part left adhering to the parent 

 shell. Moquin-Tandon says the eggs are deposited in 

 a cluster of from 50 to 60. 



Valuable notices of the genera Nerita and Neritina 

 by M. Recluz will be found in the 1st volume of the 

 'Journal de Conchyliologie / and M. Pouchet has pub- 

 lished an elaborate monograph on the Nerita fluviatUis, 

 considered in an anatomical and physiological point of 

 view. Neritina is very closely allied to Nerita, and pro- 

 bably only forms a section of the latter genus. There are 

 marine, as well as freshwater, species of Neritina. 



Neritina fluvia'tilis*, Linne. 



Nerita jluviatiUs, Linn. Syst. Nat, ed. xii. p. 1253. N. fluviatilis, F. &. 

 H. iii. p. 3, pi. Ixxi. f. 1, 2, and (animal) pi. H. H. f. 1. 



Body of a clear yellowish-grey, speckled with black above, 

 white below : head and snout black : mouth very large, fur- 

 nished with cartilaginous jaws and a lingual plate or riband, 

 which is very comphcated : tentacles clear greyish-white, darker 

 at the sides, and more or less streaked with black transversely ; 

 they diverge widely from their base, and are very slender, 

 ending in a fine point : eyes very large and black : foot obtusely 

 rounded in front, and having its extremity or tail covered by 

 the operculum when the animal is crawling. 



Shell convex above, slightly compressed towards the spire, 



* Inhabiting rivers. 



