432 
SILURIA. 
[Chap. XVIII. 
geologists), which, whilst it encircles the great Coal-fields of Michigan, Illinois, 
and the southern part of the Appalachian Coal-field, is wanting in Pennsyl- 
vania and Ohio. This rock contains many varieties of Productus, viz. P. semi- 
reticulatus, P. Cora, P. punctatus, &c, with species of Bellerophon, Goniatites, 
Spirifer, and Terebratula, known not only in Britain, hut also in many parts of 
Europe and Asia *. 
The Foraminifera, a group of which few have been determined in the Palaeo- 
zoic strata, have also a very wide range ; for the very same species (Fusulina 
cylindrica) which so abounds in Southern Russia is also found in the Carboni- 
ferous rocks of North America. 
The lamelliferous 1 Rugose ' Corals (Zaphrentis, Lithostrotion, &c.) are not 
absent ; but they are chiefly of species distinct from those of Europe ; and some 
of the forms of this group most characteristic of the old Continent (Syringopora, 
Amplexus, &c.) appear to be absent from this series in the new. 
It is unnecessary here to dilate upon the vast overlying productive Coal-fields, 
which, occupying distinct basins of stupendous dimensions, have been described 
by numerous native authors (Rogers, Owen, &c), and have been well depicted 
by Lyell and Dawson. But I must advert to the great similarity — nay, identity 
— between the Plants of these vast Coal-deposits and those of Europe. It is a 
feature highly worthy of the special notice of geologists, that at this early period 
the same species of gigantic Plants were spread over an enormous area of the 
earth's surface, which, from the nature of its vegetation, must therefore have 
been under the same conditions of atmosphere and climate, if not of physical 
outline. This specific agreement is indeed curiously at variance with the sharp 
distinction between the existing floras of North America and Europe (see 
Lyell's Travels in North America). 
This short notice of the development of the earlier fossiliferous rocks in the 
United States cannot be concluded without a special allusion to the important 
additions made to this branch of knowledge by certain American authors. The 
researches of Prof. James Hall, as recorded in his work on the Palaeontology of the 
State of New Yorkf> have shown that, with increased observation, there is great 
difficulty (except for short distances and in limited tracts) in drawing an arbitrary 
line between the lower and upper divisions of the Silurian system. The thinning 
out or the thickening of the subdivisions termed Medina Sandstone, and Clinton 
and Niagara Groups, is accompanied by just the same kind of intermingling of 
fossils which has been observed, though to a greater extent, in the British 
Isles. Several species, says Prof. Hall, known before only in the lower are now 
found at the base of the higher group. But, before we can speak of the number 
of species which range from the Lower into the Upper Silurian, all the older 
fossils of North America must be examined and compared. Even in the one 
State of New York, however, we learn that Bellerophon bilobatus, Orthis lynx, 
Strophomena alternata, and some other forms common to Europe and America, 
and abundant in the Lower Silurian, occur in the higher part of the Medina 
Sandstone, which in that country is intimately united with the upper division. 
The same elaborate work of Prof. Hall must be consulted for the figures of 
many species of Fucoids common to the Medina Sandstone last noticed and the 
associated Clinton Group. Numerous tracks or trails of Gasteropods and Crus- 
tacea also occur, indicating that the sediment so marked was accumulated in 
shallow water, or under ebbing and flowing tides, and on sloping shores. To 
* The Cestraciont (Cochliodont) Fishes of Illinois, discovered by Dr. B. Shumard, have been 
described by Dr. Joseph Leidy (Trans. Amer. Phil. Soe. vol. xi. p. 687). 
t Nat. Hist. New York, Part VI. Palseontol. vol. ii. 1852, p. 3. 
