PRBSIWICH CRAG-BEDS OP STJFFOLE: A:!^D NORFOLK. 127 



" Oceanic, or occasional visitants." " The first of these divisions 

 corresponds with the ' Arctic ' and ' Boreal ' types of Porbes and 

 Hanley ; and the second to their ' Atlantic' and ' Lusitanian ' types. 

 Their 'South-British,' 'European,' 'Celtic,' and 'British' types 

 indicate mixed or neutral ground, and partake both of northern and 

 southern characters." The high authority of Mr. Gwyn Jeffreys 

 would induce me to adopt this broad division ; but for geological 

 purposes, such as tracing the old land-margins, the old sea-areas, 

 and other questions connected with the physical geography of former 

 periods, 1 think it desirable to look at the subject more in relation 

 to existing continents and sea-margins. I have therefore retained 

 the term " Arctic " for the species living on the Spitzbergen or 

 Greenland coasts, " Scandinavian " for the species frequenting the 

 coasts of Norway, Sweden, and Denmark, " British " for those of the 

 seas immediately surrounding Great Britain, " West European " for 

 the species frequenting the coast of Europe from Prance to the Straits 

 of Gibraltar. Although Mr. Jeffreys has shown that the Mediter- 

 ranean molluscan fauna cannot be considered distinct from that 

 of the East Atlantic, I have retained the Mediterranean area as 

 a separate province, in consequence, not so much of its exist- 

 ing fauna, as of the relation of that fauna with the fauna of the 

 later or recent Tertiaries of Italy, France, and England — a relation 

 of the most marked character. Eor the species which have a more 

 southern range and are found on the shores of Madeira, the Canary 

 Isles, and the Azores I use the term " Mid-Atlantic." To this an- 

 other region is now added, embracing the great depths of the 

 Atlantic generally — depths from 1200 to 15,000 feet. 



Mr. Searles Wood described 322 species of Coralline- Crag Mol- 

 lusca, which, with the 5 species of Brachiopoda described by Mr. 

 Davidson, gives a total of 327 species. The late Dr. "Woodward, 

 basing his calculations on these lists, was of opinion that the 

 number of extinct species was 159, and of recent species 168, which 

 gave a percentage of 51 of recent species. Of the latter he con- 

 sidered that 139 were still to be found in British seas, whilst 

 27 were now confined to southern and 2 to northern seas *. 

 The great extension of the field of research by means of deep- 

 sea dredging, however, has brought to light facts which render a 

 review of the relation of the fossils of the Crag to living species desi- 

 rable. Several species which were then supposed to be extinct have 

 since been found living ; and other links have been found which 

 tend to show that some of the species supposed to be distinct may 

 be considered merely varieties of others ; and I am glad to have 



* Sir Charles Lyell's ' Antiquity of Man,' 1863, p. 209. In his ' Student's 

 Elements of Geology,' p. 178, just published (1871), Sir Charles Lyell has re- 

 vised these lists as under : — 



Percentage 

 Total Not known of shells 



number. as living, not known 



Biralves 161 47 1 asliving. 



Univalves 184 60 )■ 31-5. 



Brachiopods 5 3 



