Chap. XIV.] 
UPPER SILURIAN IN RUSSIA. 
359 
serling, all the strata (about 1500 feet thick) ' are but leaves in one united cal- 
careous volume/ no distinctions can be drawn except by close examination of 
the organic remains. 
The chief mineral masses of these overlying rocks are fine-grained limestones 
(passing in some beds almost into marble, and thus presenting a contrast to the 
incoherent and earthy Lower Silurian limestones of St. Petersburg), and mag- 
nesian beds (dolomite) occasionally somewhat crystalline and cavernous, here 
and there charged with white siliceous concretions. In other parts the strata 
are friable, marly, and of grey and yellow colours. 
Rocks having the same mineral aspect, and containing the same fossils, ap- 
pear, as I am informed by Keyseiiing, along the edge of the Timan Hills, in the 
distant region of the Petschora, at the mouth of the River Gatchina, and near 
the shore of the Icy Sea. It is further to be remarked Jhat neither there nor 
in Esthonia (that is, in tracts 1000 miles distant from each other) has there been 
detected the slightest unconformity between the Lower and Upper Silurian 
rocks. 
The task of working out the organic remains as respects Esthonia and the 
adjacent Islands of Wormes, Dago, Oesel, &c. has fortunately been accom- 
plished since the first edition of this work was published, by Professor Schmidt 
of Dorpat, whose views, as communicated to me by my associate Keyserling, 
have been published*. The coralline limestone forming the summit of the 
Lower Silurian series graduates in the same cliffs into a more marly lime- 
stone with Pentamerus linguifer, Strophomeua pecten, and other fossils. 
The upper portion of the band is everywhere characterized by a profusion of 
Pentamerus borealis, another species (probably Pentamerus oblongus) being 
partially distributed ; whilst the associated fossils are Calymene Elumenbachii, 
Encrinurus punctatus, Leptsena depressa, Atrypa reticularis, Leperditia Balthica, 
and L. marginata, with many Corals. Next follows a bluish-grey marlstone, 
in part a hard subcrystalline dolomite, whose chief fossils would seem to refer 
the rock to the Wenlock age, such as Lichas Gotlandicus, Orthoceras annu- 
latum, Orthis elegantula, Leptsena tracsversalis, &c. This zone also contains 
many other Upper Silurian species. 
The uppermost Silurian rock (as developed in the Isles of Oesel, Dago, and 
Moen) is a crystalline limestone, rarely dolomitic, the beds of which alternate 
with marly shale. This deposit contains many Mollusks found also in the 
Ludlow rock, such as Pterinea reticulata, Grammysia cingulata, Orthoceras 
bullatum, Murchisonia cingulata, Platychisma helicites, Rhynchonella Wilsoni, 
and Orthis orbicularis, together with Eurypterus remipes ? (or E. tetragonoph- 
thalmus) — a representative of the group of Crustaceans which so much abound 
in the Ludlow rocks. Again, the resemblance of the Russian to the British 
deposit is marked by the first appearance of Fishes in the ascending order of 
deposits. Of the genus Cephalaspis, which in England first appears in this 
zone, the C. verrucosus of Pander (Thyestes, Eichw.), and C. Schrenkii, Pander, 
occur in the Isle of Oesel, together with Onchus Murchisom, which is found also 
near Ludlow (see p. 138, and PI. XXXV. f. 13, 14) f. These Uppermost Silurian 
* Quart. Journ. G-eol. Soc. vol. xiv. p. 43. canthus, Onchus. — The teeth are named Aula- 
t In addition to the Cephalaspides (of several codus (Sphagodus, Eichwald), Strosiphorus, Odon- 
species), Professor Pander describes the following todus, G-omphodus, Coscinodus, Monopleurodus. 
genera of Upper Silurian Pishes, mostly Ganoids, If, by further comparison, several of these genera 
viz. Eytidolepis, Schidiosteus, Coccopeltus, Cy- should disappear, the researches of Dr. Pander 
phomalepis, Trachylepis, Stigmalepis, Dasylepis, will still have made a very considerable addition 
Lopholepis, Dictyolepis, Oniscolepis, Phlebolepis, to Upper Silurian Ichthyology. It remains, how- 
Melittomalepis, Tolylepis, Lophosteus, Pterich- ever, to be determined to what extent some of 
thys, Coelolepis, Pachylepis, Nostolepis. The these uppermost beds in Oesel may not represent 
Ichthyodorulites are Ehabdacanthus, Priona- the base of the Devonian rocks. 
