MOLLUSCS NON-MARINE Owing to the scarcity of calcareous soils, the paucity of woods and quiet streams and the fouling of many of the rivulets by the waste products of the mines and china-clay works, Cornwall is not a county favourable to the development of terrestrial and fluviatile mollusca. Still out of a total of 139 species known to occur in the British Islands 82 have been recorded from the duchy. This does not take count of the following species, which have been cited, but seemingly in error, viz. Punctum pygm&um, Buliminus montanus, Pupa anglica, Vertigo substriata^ Clausilia triplicate^ Planorbis marginatus and Sphcerium lacustre. Some of these forms, and sundry others, may however yet be found when more thorough search has been made than has hitherto been the case. The most interesting member of the fauna is Hygromia reve/afa, whose distribution in the British Isles is confined to south Devon, Corn- wall, the Scilly and Channel Islands. Like Helix pisana, which occurs in west Cornwall, Hygromia revelata is a south-western form, and seemingly a survivor of the fauna which inhabited the old lowlands, now sub- merged, that in Pleistocene times extended along the west coast of Europe down to Portugal. In this connection the recent discovery 1 in the Neolithic burial ground at Harlyn Bay of abundant examples of Hygromia montivaga (West.), a species closely allied to H. revelata, but now only found living in Spain and Portugal, becomes of the highest interest ; and one would not be greatly surprised were living examples of the Irish slug (Geomalacus maculosus^ Allman), which is itself another survivor of the old lowland fauna, to be discovered some day in one or other of the little wooded valleys around the Cornish coast. A handsome and striking variety of the big black slug (Arion ater] seems common in the county. This instead of being entirely black has a broad yellow stripe along either side, and it has in consequence had the varietal name of bicolor bestowed upon it. The Cornish streams being as a rule both short and rapid the larger freshwater molluscs are nowhere to be met with, save the freshwater pearl mussel (Unio margaritifer] , which prefers these waters, and which has been taken in the Camel, the Tamar and other streams from the hilly districts. Roughly speaking this species is confined to the area north and west of a line joining Plymouth and Hull. The pearls from 1 Pnc. Malac. Soc. Land. v. 1 8 8. 160
