222 PALEOZOIC TIME — LOWER SILURIAN. 



3. Articulates. — Among Trilobites, the Asaphus (Isotelus) gigas, and Trinu- 

 cleus conccntricns, continue on from the Trenton period ; but the A. gigas is rivalled 

 both as to abundance and size by the A. megistos, already referred to, found in 

 Ohio and other States west. The Triarihrus Beekii is common in the Utica 

 shale, and occasionally seen in the Trenton beds. The head-shield generally 

 occurs without the body : fig. 354 represents its usual form, and fig. 355 the 



Figs. 354, 355. 



Triarthrus Beekii. 



same entire. The body is much like that of a Calymene (fig. 321) : it has a row 

 of minute spines along the middle of the back. 



III. General Observations on the Trenton and Hudson Periods. 



Geography. — The Trenton and Hudson periods stand apart less 

 through a difference of fauna than a geographical change which 

 caused, over large areas, shales to succeed limestones. Were the 

 life alone considered, they might well be united in one period. 



Since the Trenton limestones extend widely over the continent 

 and are full of marine fossils, the land must have been covered as 

 widely by the sea. The beds reach nearly to the Azoic on the 

 north, and hence the coast-line from New York westward was 

 situated but little south of its position in the Azoic period. (See 

 maps, pp. 136, 170.) And it is probable, from the occurrence of 

 the rocks in the Winnipeg basin, that this line, bending northward 

 near Lake Superior, followed the western border of the Azoic 

 towards the Arctic. 



The general absence of shales, even along the northern border 

 as well as the Appalachian region, can be accounted for only on 

 the supposition that the ocean had not free access from the east- 

 ward over the continent. The interior was probably in the condition 

 of the lagoon or inner basin of a coral island. A barrier produced 

 by the slight emergence of some part of the Atlantic border 

 would have caused such a condition of the continent; and less 

 than ten feet of elevation would have been sufficient, — for no more 

 is needed in the islands of the Pacific. Its breadth may have varied 

 from scores of miles to only a few rods. To the south, the waters 

 probably opened (as they did afterwards) into the Gulf of Mexico: 

 a connection with the ocean was necessary for their purity. 



