But besides the above-named species there are a considerable number of well-marked varieties of shells which seem to be peculiar to our islands. A list of these has been kindly furnished me by Mr. Theo. D. A. Cockerell, who has paid much attention to the subject; and after omitting all those whose peculiarities are very slight or whose absence from the continent is doubtful, there remain a series of forms some of which are in all probability really endemic with us. This is the more probable from the fact that an introduced colony of _Helix nemoralis_ at Lexington, Virginia, presents numerous varieties among which are several which do not occur in Europe.[84] The following list is therefore given in the hope that it may be useful in calling attention to those varieties which are not yet positively known to occur elsewhere than in our islands, and {357} thus lead, ultimately, to a more accurate knowledge of the facts. It is only by obtaining a full knowledge of varieties, their distribution and their comparative stability, that we can ever hope to detect the exact process by which nature works in the formation of species. LIST OF THE SPECIES AND VARIETIES OF LAND AND FRESHWATER SHELLS WHICH, SO FAR AS AT PRESENT KNOWN, ARE BELIEVED TO BE PECULIAR TO THE BRITISH ISLES OR NOT FOUND ON THE CONTINENT. LIMACIDAE. 1. Limax marginatus, _var._ MACULATUS. Ireland; frequent, very distinct. 2. ,, ,, ,, DECIPIENS. Ireland and England. 3. ,, flavus, _var._ SUFFUSUS. England; Melanic form. 4. ,, ,, ,, GRISEUS. England; Melanic form. 5. Agriolimax agrestis, _var._ NIGER. Yorkshire. Melanic. Azores. 6. ,, ,, ,, GRISEUS. England. Melanic. 7. Amalia gagates, _var._ RAVA. W. of England. 8. ,, sowerbyi, _var._ RUSTICA. England. 9. ,, ,, ,, NIGRESCENS. Surrey and Middlesex. 10. ,, ,, ,, BICOLOR. Ealing. 11. Hyalina crystallina, _var._ COMPLANATA. Near Bristol. 12. ,, fulva, _var._ ALDERI. 13. Vitrina pellucida, _var._ DEPRESSIUSCULA. S. England, Wales. HELICIDAE. 14. Arion ater, _var._ ALBO-LATERALIS. England, Wales, Isle of Man; very distinct. 15. ,, hortensis, _var._ FALLAX. England. Common at Boxhill. 16. GEOMALACUS MACULOSUS. Kerry and Cork. Three varieties have been described, one of which occurs in Portugal. 17. Helix aspersa, _var._ LUTESCENS. England. Not rare perhaps in France. 18. ,, nemoralis, _var._ HIBERNICA. Ireland. 19. ,, rufescens, _var._ MANCHESTERIENSIS. England. 20. ,, hispida, _var._ SUBGLOBOSA. England. 21. ,, ,, ,, DEPILATA. England. 22. ,, ,, ,, MINOR. England, Ireland. 23. ,, granulata, _var._ CORNEA. Lulworth, Dorset. 24. ,, virgata, _var._ SUBAPERTA. Bath. 25. ,, ,, ,, SUBGLOBOSA. England, Wales, Bantry Bay.