24 NERITID^. 



Inhabits rivers, and lakes which are supplied with 

 running water, especially where the bottom is gravelly, 

 in many parts of Great Britain, but not everywhere. 

 Gwyn Jeffreys remarks that Neritina " is very closely 

 allied to Nerita',' a genus of marine molluscs, of which 

 it ''probably only forms a section," and that "there 

 are marine as well as freshwater species of Neritina^ 

 He found Neritina fluviatilis in Loch Stennis, Ork- 

 ney, with the marine My a arenaria. The Rev. J. 

 McMurtrie has very kindly furnished me with the 

 following particulars respecting the above-named 

 locality : — " Loch Stennis communicates with the sea, 

 and the lower reach is salt, becoming only slightly 

 brackish at the constriction where the loch is divided 

 into two reaches. I found a few specimens in com- 

 paratively salt water, but at and above the con- 

 striction, where the water is only very slightly 

 brackish, Neritina becomes abundant. The shells are 

 clean and beautifully marked." Claparede states 

 that the female deposits her capsules upon the shell 

 of her neighbour, not upon her own, or more rarely 

 upon stones or the shells of other molluscs. Each cap- 

 sule contains from forty-five to sixty eggs, but only a 

 single embryo is developed, the rest of the eggs being 

 devoured by the young Neritina before it emerges 

 from the capsule. The capsules, which are often 

 mistaken for the eggs, are globular, slightly flattened on 

 one side, and enclosed in a tough covering, consisting 

 of two segments, which are firmly united together at 

 first, but split asunder when the embryo is excluded, 

 the upper half falling off, while the lower one is left 

 adhering to the object to which it was attached. 



