262 



USE OF "GENERIC AREAS." 



there are as many as seven species. Of these, one 

 at least (M. ebena) is a well-known Mediterranean 

 form ; so that at that time the range of the genus 

 was more northern than it is at present. 



Mitrce occur on the Pacific coasts of America, 

 but are seemingly wanting on those of its Atlantic 

 border ; but far to the north, in the seas of Green- 

 land, there is a solitary form {Mitra Groenlandica), 

 a seemingly exceptional case in the distribution of 

 the genus. It is, however, just the kind of exception 

 which serves to show the reality of generic centres. 



This Greenland Mitra occurs fossil in Ireland, in 

 association with another species of the genus (M. 

 cornea) now living in the south Lusitanian zone, 

 and it thus becomes linked with its congeners. It 

 remains, there, to attest the furthest extension of its 

 race, and is to the zoologist, when speculating on 

 the former range of lower tribes of animals, just 

 what the lonely Runic pillars, on the same Green- 

 land coasts, are to the antiquarian, when engaged in 

 tracing out the remote settlements of the early 

 Northmen. 



The palaeontologist may derive much useful 

 guidance from the study of generic areas. They 

 will often enable him to determine the extent to 

 which old seas may have been connected, whilst the 

 occasional isolation of any definite forms, by inter- 

 vals of deep and broad sea, is to him direct evi- 

 dence of the former continuity of conditions, along 

 which such forms have travelled — of physical 



