160 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ Dec. 1, 
each country to one formation : im this will be found many common 
species, as 
Leptzena sericea. Terebratula cuneata. 
Orthis testudinaria. Stricklandi. 
--—— parva. Spirifer crispus. 
elegantula. Pentamerus levis. 
Terebratula bidentata. oblongus. 
Such instances might be multiplied to a greater extent; but the 
above are sufficient to show that the vertical range of each species is 
nearly the same in both these distant countries. 
It would be interesting to trace out the first appearance of each 
species in many countries, and to see whether it is found in one at an 
earlier period than in another country ; and thus learn of what region 
it was originally native. At present we have slight materials for 
such an inquiry ; but some facts have been met with while following 
the present investigations, which bear upon the subject. 
Spirifer Urii, which in Britain is found sparingly m South Devon, 
more abundantly in the S. Petherwin and Pilton beds which unite 
the Devonian and carboniferous formations, and is most common in 
the shales of the carboniferous period, is in the United States rather a 
Devonian than a carboniferous species; appearing first in the Oriskany 
sandstone, a bed which lies next above the Wenlock formation. 
Terebratula reticularis is not known here below the Wenlock 
shale: in New York it is common in the Lower Silurian Clinton 
group ; and if a single doubtful specimen may be trusted, occurs low 
down in that series in the blue limestone of Cincinnati. 
Orthis resupinata is rare with us in the Devonian beds, and com- 
mon in the mountain limestone: in the United States it is found as 
low as the Upper Pentamerus limestone, where all its companions 
belong to the Wenlock epoch. It is impossible to be mistaken about 
so well-marked a shell; but had this been found alone, or m com- 
pany only with unknown species, no European geologist would have 
hesitated to class the bed in the carboniferous series. 
All the species just named are found in North America in beds of 
an older date than those in which we find them here, and must there- 
fore be set down as native Americans which have migrated at a later 
period to Europe. . When we know the American shells better, we 
may be able to point out European species which had established 
themselves here before they passed to the United States; perhaps 
Leptena depressa may be one of them. 
Leptena depressa is abundant in the schists connected with the 
Bala limestones, and continues throughout all the beds from that to 
the mountain limestone. The Bala limestone is certainly low down 
in the Lower Silurian formation ; and in Professor Sedgwick’s opinion 
it lies below all the Silurian beds described by Sir R. Murchison. 
In the United States the Leptena depressa is found but sparingly in 
the Trenton limestone and blue limestone of Ohio which he im the 
middle of the Lower Silurian series, and is first found in any abun- 
dance at the top of that series in the Clinton group. 
If, without regarding species, we look only to general form, we 
