NERITINA. 53 



mantle. They have not been observed to float_, 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 fluviatilis, 

 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 fluviatilis, 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 jellowish-grey, speckled with black above, 

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

 nished with cartilaginous jaws and a Ungual plate or riband, 

 which is very complicated : 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. 



