v.] INTRODUCTION. XCi 



southern limit of the one^ and the Bay of Biscay as the 

 northern limit of the other area. The greater part are 

 common to the North and South. In considering the 

 Crag Mollusca^ the percentage of existing or recent spe- 

 cies would be very much larger if we were to include 

 the Bed Crag and the Mammaliferous or Norwich beds, 

 and especially if we were to add the pleistocene or post- 

 pliocene strata which immediately overlie those beds — 

 in fact the whole of our upper tertiaries. It is highly 

 probable that all the Mollusca which lived during the 

 periods represented by the newer strata still survive in 

 some part or other of those vast tracts of sea-bed which 

 lie between the North Pole and the Pillars of Hercules. 

 The discovery which is continually being made of missing 

 links, as well as the increase of experience which results 

 from a more extensive and perfect knowledge of the Mol- 

 lusca, must tend to alter the rate of percentage as between 

 recent and fossil forms. I am aware that the late Pro- 

 fessor D^Orbigny (in his ^ Paleontologie Fran9aise^), Pro- 

 fessor Agassiz (in his ^ Essay on Classification^), as well 

 as Hall, Pictet, and others, have contended that there 

 is no specific identity between any of the Tertiary and 

 recent Mollusca ; but the peculiar views which some of 

 those naturalists entertained and advocated, as to the 

 successive creation of species, may have influenced their 

 judgment. At all events, he must be a bold species- 

 maker who can pretend to distinguish Crag specimens of 

 the common European cowry, and of many other species, 

 from those which now live in the adjacent seas ; and their 

 varieties and monstrosities also, both in a fossil and recent 

 state, coincide in the most minute particulars, the only 

 difierence being that the latter are glossy and compara- 

 tively transparent, while the former are dull and opaque. 

 Even the lAngula of the Wenlock Silurians could not be 



