DIFFUSION OF THE CAMBRIAN FAUNAS. 15 



VIIL— ObtHOCERAS ' — DlCRANOGRAPTU^. 



There remains still another graptolitie fauna among the older I'ala'ozoic deposits in 

 North America to which I would ask your attention. This is that of the Utica slate. 



After the irruption of the Arenig graptolites into the valley of the St. Lawrence, this 

 depression, then a sound of the sea, was again elevated and the cold waters of the north 

 excluded, so that the Trenton fauna was enabled to extend itself through the A^alley, at 

 the same time that it occupied extensive tracts to the west and south-west. 



Subsequent to this we may suppose that a period of depression set in and the areas 

 in the St. Lawrence Valley where the Trenton fauna had flourished was invaded by a new 

 fauna, that of the Utica slate. This fauna succeeded in extending itself further west 

 than its predecessors of the Atlantic coast containing graptolites. Beside occupying the 

 St. Lawrence Valley it was spread westward across the provinces of Quebec and Ontario, 

 and southward through New York and Pennsylvania. The greatest depression of this 

 period, if the thickness of the measures be taken as a criterion, was probably in the latter 

 state, where the deposit attains a thickness of 400 feet, though near Quebec it was 300 

 feet.- This, however, does not give the measure of de^jth of the sea, which probably was 

 much greater. 



At Cincinnati in Ohio, the Utica slate is wanting and also further west, where the 

 Hudson Eiver beds rest directly upon the Trenton. A part of the Hudson River Group 

 thus corresponds in age to the Utica slate of eastern localities. In the Triarthri of the 

 Utica slate, we have one of the last surviving genera of the Cambrian age. At the close 

 of Cambrian time appeared in Europe Triarthnis Angelini ; at the opening of Ordovician 

 time T. Fischeri lived in America in company with the graptolites of the Quebec Group : 

 when again the valley of the St. Lawrence was invaded by a graptolitic fauna, the typical 

 Triarthri appeared. 



This comparison of the genera on the two sides of the Atlantic at this epoch of geo- 

 logical time might be carried further. Especially might we note the slow translation of 

 the Orthocerata and of many gasteropods from America to Europe, but enough has been 

 said to suggest the line of investigation which could be pursued. A feature which stands 

 out most prominently is the slow migration of the shallow-water forms when compared 

 with those which we regard as inhabitants of the deeper and colder seas. No sooner do 

 the latter appear in Europe than almost simultaneously we iiud them (or species closely 

 related to them) on the Atlantic coast of the New World. 



IX. — Tabular View of Migrations of Genera. 



In order to present a synoptical view of the migrations of genera in early Palteozoic 

 time the following table has been prepared. The genera named in it are chiefly those 

 Avhich have been referred to in the preceding paper. 



The central column shows the several graptolitic faunas indicated by one selected 

 genus ; the other columns show the chronological relation of other Cambrian and Ordo- 

 vician genera to the graptolites. A double horizontal line divides the Cambrian from the 



' Orthoceras laqueatum. ^ These thicknesses are excessive. 



