40 INTRODUCTION. 



It may be well to observe, that the fossil shells now 

 found in the different strata show that a different 

 geographical distribution of these animals existed in 

 a former state of the globe ; for several genera were 

 found in this country then which are now confined 

 to warmer climates. Thus, there are in the most re- 

 cent strata, mixed with existing recent shells, remains 

 of species which agree with those now only found in 

 other parts of Europe and the north of Africa. 



Mr. Morris, for example (Mag. N. Hist., 1836. 

 262. n. s. ii. 544.), has recently found the follow- 

 ing 36 species of recent British shells, along with re- 

 mains of Mammalia, at Grays, Erith, Copford, Sut- 

 ton, and Ilford, on the banks of the Thames, not very 

 far from London. 



1. Limax , species not determined. 



2. Succinea amphibia. 



3. Pfeifferi (oblong a). 



4. Helix hortensis. 



5. rufescens. 



6 . paludosa. 



7. hispida. 



8. trochiformis. 



9. fusca. 



10. Zonites lucidus. 



11. Zua lubrica. 



12. Pupa marginata. 



13. sexdentata. 



14. Carychium minimum. 



15. Limnaus auricular is. 



16. pereger. 



17. truncatulus. 



18. glaber. 



