, MOLLUSCS 



NON-MARINE 



Durham is not a county in which the non-marine mollusca find 

 conditions suitable for their abundant development. 



In the large tract of Magnesian Limestone that extends from South 

 Shields to Hartlepool along the coast, and is bounded on its inland exten- 

 sion by an almost straight line from the latter place to Darlington, and by 

 an irregular line from South Shields to Gainford (about seven miles west 

 of Darlington), there are numerous valleys that produce a considerable 

 number of land species. To the west, however, though the land surface 

 is a good deal diversified, it is on the whole too hilly to afford suitable 

 habitat. 



The small extent of marshes and ditches and the absence of canals 

 or slow-running rivers account for the fact that the freshwater species are 

 much less abundant here than in the more southern parts of England. 



Still, out of 140, or so, species met with in the British Islands, 

 94 have been recorded for Durham, nor is this number likely to be 

 much increased by further research. 



The most interesting form is Limax tenet/us, Mull., which was first 

 described as British from a specimen procured in a wood at Allansford. 

 It was generally supposed for some time that the individual so identified 

 was merely the young of some other species ; quite recently, however, 

 this slug has been re-discovered in several localities in the British Isles. 



Certain species that have been chronicled are excluded from the 

 list. Helix lucida is an old record for a form of Vitrea, usually V. alliaria, 

 the true V. lucida being until lately unknown to our conchologists. Unto 

 pictorum and Planorbis vortex were recorded by Hogg (in Brewster's 

 History of Stockton-on-Tees, 1827), but these identifications are doubt- 

 ful. Similar uncertainty attaches to the record of L. brunneus, Drap., 

 which was said to be frequent in damp woods. Dead shells of Vivi- 

 para vivipara and Neritina Jiuviatilis have been met with on the coast, but 

 have evidently been brought in ballast by ships. 



Pomatias elegans is found in Yorkshire, and has been recorded for 

 Northumberland, so that its absence from Durham is noteworthy. Heli- 

 cella cantiana, although included in our list, is not common, and is by 

 some suspected to be a latter-day introduction, but then it has as yet not 

 been found in the fossil state anywhere in Britain. 



With the exception of this last-named species there is an absence of 

 all continental and south-western (or Lusitanian) forms, so that the assem- 

 blage is of the normal north-British type. 



The literature of the subject is not very extensive, and mostly 

 scattered, the two more important papers being that by J. Alder (the 

 discoverer of several, and author of four British species) in the Trans- 

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