MY A. 35 



truncated at tlie hinder part. It sometimes attains the size 

 of four inches long and two and a half broad. The animal 

 burrows to the depth of a foot or more, communicating 

 with the surface of the sand or gravel by means of a hole, 

 through which, when disturbed, it will throw out a jet of 

 water in seeming fury. The following remarks on the de> 

 formity and degeneration occasioned in some specimens of 

 this species by a change in the element by which they are 

 surrounded is found in Forbes and Hanley^s 'History of 

 British MoUusca \ — " The M^a arenaria is occasionally 

 found in brackish water, and is there subject to dwarfing 

 and distorting. Such is the condition of the specimens in 

 the Loch of Stennis, in Orkney, famous for the part it j^lays 

 in the scenery of Scott's admirable novel of ' The Pirate.' 

 In that lake we find Limnei, Neritince, and other fresh-water 

 molluscs, along with the M?/cej which now however appear 

 to be nearly if not altogether extinct. Before they became 

 so, they had greatly diminished in size, and become vari- 

 ously distorted. In this instance, the cause is to be sought 

 for in a very recent elevation of the land, which has gra- 

 dually converted what was originally an arm of the sea into 

 a brackish pool, only occasionally flooded with salt water, 

 and probably destined eventually to become a fresli-water 



