ON BRITISH PULMONIFEROUS MOLLUSCA. 139 



what more numerous than elsewhere. Some rare species, 

 however, occur, with several of the scarcer forms ; Helix 

 fulva, scarburgensis and excavata, have lately been added to 

 the list by Mr. Alder. Helix crystallina is not uncommon ; 

 Helix ericetorum and Bulimus acutus abound on sandy soil in 

 the outer Hebrides. Pupa palustris and Helix radiatula 

 were found on the islands opposite Oban by Mr. Jeffreys, and 

 Pupa cylindrica in Skye (on trap) by Mr. Macaskill. On 

 the mountains Helix alliaria is not rare. 



District X. — Only five species oi Pulmoniferous Mollusca in- 

 habit the Shetland Isles. These five are Arion ater, Limax cine- 

 reus, Vitrina pellucida, Helix alliaria and Limneus pereger^ 

 all species common to the whole of the north of Europe, and 

 extending their range to Greenland. The geological struc- 

 ture of these islands being primitive is unfavourable to the 

 development of species ; but I regard the distribution as 

 wholly climatal in this, the most northern province, inasmuch 

 as it is in no way influenced by the tracts of limestone which 

 occur in certain localities in Shetland. Individuals are as 

 scarce as species ; the only animal of those enumerated at all 

 plentiful is Avion ater. 



In the first of the two following tables the numbers of the 

 species of eachgenusfoundin the various districts are exhibited. 

 For the first province the materials were derived from per- 

 sonal research ; for the second, from published lists and com- 

 munications from Mr. Jenyns and Dr. Stanger ; for the third 

 and fifth, from published lists and personal observation ; for 

 the fourth, from the published lists of Mr. Alder, and communi- 

 cations from that gentleman, Mr. Bean and Dr. Greville. The 

 sixth and seventh districts yielded their numbers through the 

 medium of my friend Mr. Thompson, of Belfast ; the eighth 

 and ninth from personal observation, published lists, and 

 communications from Sir William Jardine, Dr. Johnston, and 

 Mr. Smith of Jordanhill, and other gentlemen. The tenth I 

 investigated myself. 



In the second table, the distribution of the species on the 

 various strata is exhibited. If the pectinibranchons Mollusca 

 had been added, the preponderance of the cretaceous and 

 oolitic strata would have been much more evident, and there 

 are several species at present known entirely as inhabitants of 

 the carboniferous rocks, which, I doubt not, will also be found 

 on the former, or have been confounded with other species. 



In both tables I have omitted all forms which I could not 

 regard as true species, or the indigenousness of which has 

 been questioned on good grounds. 



