G. 5.--Like the south, the eastern, western, and northern districts of England were cut off from the centre by natural barriers. The Fens of Cambridgeshire and the marshes of the Lea valley, together with the dense forest along the "East Anglian" range, enclosed the east in a ring fence; within which yet another belt of woodland divided the Trinobantes of Essex from the Iceni of Norfolk and Suffolk. The Severn and the Dee isolated what is now Wales, a region falling naturally into two sub-divisions; South Wales being held by the Silurians and their Demetian subjects, North Wales by the Ordovices. The lands north of the Humber, again, were barred off from the south by barriers stretching from sea to sea; the Humber itself on the one hand, the Mersey estuary on the other, thrusting up marshes to the very foot of the wild Pennine moorlands between. And the whole of this vast region seems to have been under the Brigantes, who held the great plain of York, and exercised more or less of a hegemony over the Parisians of the East Riding, the Segontii of Lancashire, and the Otadini, Damnonii, and Selgovae between the Tyne and the Forth. Finally, the Midlands, parcelled up by the forests of Sherwood, Needwood, Charnwood, and Arden, into quarters, found space for the Dobuni in the Severn valley (to the west of the Cateuchlani), for the Coritani east of the Trent, and for their westward neighbours the Cornavii.[48] G. 6.--All these tribes are given in Ptolemy's geography, but only a few, such as the Iceni, the Silurians, and the Brigantes, meet us in actual history; whilst, of them all, the Damnonian name alone reappears after the fall of the Roman dominion. Thus the accepted allotment of tribal territory is largely conjectural. North of the Forth all is conjecture pure and simple, so far as the location of the various Caledonian sub-clans is concerned. We only know that there were about a dozen of them; the Cornavii, Carini, Carnonacae, Cerones, Decantae, Epidii, Horestae, Lugi, Novantae, Smertae, Taexali, Vacomagi, and Vernicomes. Some of these may be alternative names.