XXXIV INTRODUCTION. 



border on tho Mediterranean, participate in the presence 

 of such of our species as form part of the fauna of that 

 region . 



In alpine regions, of higher elevation than those that 

 diversify the surface of the British Islands, a distinct and 

 positive distribution in altitude of land and fresh-water 

 Mollusca may be clearly made out, peculiar species appear- 

 ing for the first time in definite zones of elevation. On our 

 mountains, deficiencies only can be observed ; they can 

 boast of no peculiar types. The few !Mollusks that are 

 found on their higher portions are such as may be col- 

 lected in the northernmost regions of Europe ; but, at the 

 same time, are equally members of the fiiuna of our low 

 districts, and the neighbourhood of our shores. 



Whilst the early history of our naked and unprescrvable 

 Mollusks has perished without a record, the genealogy of 

 not a few of our testacea may be traced unerringly in fossili- 

 ferous formations. The area of the British Islands exhibits 

 a long series of ancient seabeds that tell unmistakably of 

 the changes that have taken place within its bounds. By 

 far the greater part of these show, in their contents, no 

 instances of specific identity with our existing Mollusca, and 

 the farther we recede in time, the more different was our 

 submarine population. The first approach to an existing 

 British species is seen in the Terebratula striatula, of the 

 upper cretaceous strata, a form scarcely distinguishable in 

 essential features from the Terebratula caput-serpenfis. It 

 is not, however, until we seek among our tertiary strata, 

 that we find true homologues of our living Mollusca. Some 

 three or four Eocene shells, especially forms of Eulima and 

 Cyliclina^ come exceedingly close to recent species, and are 

 possibly identical. There is a doubt, however ; the forms 

 themselves being what are termed critical types, and their 



