XC INTRODUCTION. [CH. 



which exist between this ancient Molluscan fauna and 

 that which at present inhabits our seas and coasts^ I have 

 not only examined the Crag strata in company with 

 Mr.Prestwich^ whose experience in this important branch 

 of geological science is so w ell known, but I have also 

 carefully gone over the extensive collection of Crag shells 

 made by Mr. Searles Wood and presented by him to 

 the British Museum. In pursuing the latter examina- 

 tion, I compared the collection with the valuable and 

 elaborate work of Mr. Wood, published by the Palseon- 

 tographical Society, in which the specimens were de- 

 scribed and figured, as well as with Mr. Davidson's 

 memoir on the Tertiary Brachiopoda in the same series 

 of publications ; and I afterwards collated the result of 

 this examination with a great many books and special 

 treatises on the recent conchology of Europe and the 

 Arctic regions. I likewise derived no small assistance 

 in the investigation from the opportunity I had of 

 consulting the large collection of recent shells in our 

 National Museum, and for which I would here return 

 my best thanks to Dr. Baird, the courteous and able 

 Curator of this department. This examination has satis- 

 fied me that, out of 386 species of marine shells belong- 

 ing to the Coralline Crag formation, no less than 167 are 

 identical with those which still live in the British seas. 

 Of the remaining 119 species, 7 are said to be exclusively 

 Northern, and 19 Southern forms, while 93 appear to 

 be extinct or are as yet unascertained to be existing. 

 This gives a proportion of very nearly 60 per cent, for 

 those marine species of the Coralline Crag which at pre- 

 sent inhabit our seas. Out of the 167 species which I 

 have recognized as British, 27 have been described or 

 recorded by different authors as Northern, and only 24 

 as Southern forms, — taking the Arctic circle as the 



