Wt 
TWENHOFEL: EXPEDITION TO THE BALTIC PROVINCES. 31 
exposure of the lower beds of the Silurian at that place. As will be 
shown later, however, there is an extensive time break at the base of 
the Silurian. 
The lithology of the Silurian is somewhat different from that of the 
Ordovician, in that there is a greater degree of dolomitization of 
the limestones, coralline limestone constitutes a larger proportion of the 
rock, and, except in the higher beds, shales are far less conspicuous. 
The faunas are quite different. The corals in many species are the 
same as those of the Ordovician and some of the Borkholm Bryozoa also 
appear to cross the system boundary; but the gastropods, pelecypods, 
cephalopods, brachiopods, and trilobites are almost altogether different. 
One can hardly fail to escape the conclusion that the Silurian de- 
posits of Baltic Russia are those of quite shallow water and near a 
shore. The horizontal variation of sediments, the extensive reef 
deposits made by animals which must certainly have lived near the 
surface, and the masses of shell breccia, as in the Borealis banks, can 
hardly lead to another view. If this view be correct it follows that 
there should be considerable horizontal lithic variation and this ap- 
pears to obtain. 
There are certain features of the faunas which are of interest. These 
features arise from the fact that common species of other parts of the 
Baltic are not found in Russia. The lowest beds of Gotiand, just a 
short distance away, are characterized by an abundance of Strick- 
landinia lirata (Sowerby), while Bilobites bilobus (Linné) of slightly 
higher beds is an equally abundant Gotland fossil. Coelospira hemi- 
spherica (Sowerby), is a common Middle Llandovery species of the 
Kristiana region of Norway and of England and it also is not found in 
the Silurian of Esthonia. 
It is not clear how the absence of these species is to be explained. 
One explanation would be that the equivalents of the Gotland beds 
containing Stricklandinia lirata and Bilobites bilobus were never de- 
posited in Baltic Russia, and that beds are also absent which are the 
equivalents of those of the Kristiana region of Norway carrying 
Coelospira hemispherica. Since the first two mentioned fossils belong 
to different horizons in the Gotland section, the above explanation 
leads to the assumption of two time-breaks in the Esthonian Silurian 
at the places where these fossils should be present; that is, somewhere 
within the Addifer and St. Johannis formations. 
The preceding explanation is by no means satisfactory. Marine 
animals have their habitats determined by a combination of an ex- 
tremely great number of varying conditions, and the absence of the 
