PRESENT HISTORY AND DISTRIBUTION 63 
Of these, the Boreal is perhaps the most peculiar, 
since it extends on the American side from Labrador 
to Cape Cod, and, crossing the Atlantic, includes the 
southern shores of Iceland and the west coast of 
Norway up to the North Cape. 
When, however, it is borne in mind that at no 
very remote period of the earth’s history there was 
a land barrier shutting the Arctic off from the 
Atlantic Ocean, it becomes obvious that the fauna 
that occupied the southern side of that barrier must, 
on its rupture, have been acted on by the contending 
currents of cold and warm water thus brought into 
contact. Hence it has in part been driven to the 
southward along the American coast by the cold 
northern waters that flow down past the western side 
of Iceland, and in part carried far to the north on the 
eastern side by the warm waters of the Gulf Stream. 
It is impossible without giving long and tedious 
lists to adequately define the several faunas of these 
provinces, but it may be generally stated that such 
genera as—* Margarita, Lacuna, Velutina, *Trichotro- 
pis, Buccinum, * Neptunea, Liomesus, *Tvophon, * A dmete, 
Bela, *Yoldia, *Modiolaria, *Astarte, Cyprina, Mya, 
Cyrtcdaria, and Lyonsia, are examples of the forms 
met with in colder regions, those marked with an 
asterisk (*) occurring in both northern and southern 
hemispheres, while Nervita, Rostellaria, Pterocera, 
Cypraa, Septa, Cancellaria, Voluta, Oliva, Marginella, 
Harpa, Terebra, Conus, Perna, Vulsella, Spondylus, 
