316 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
species mentioned may as well have been due to the absence of some 
of the conditions that were necessary for their presence. This expla- 
nation is in keeping with the fact that Bilobites bilobus is not universally 
distributed in the Gotland division in which it occurs, while in Norway 
Kiaer assigns it to different horizons in different facies. 
The divisions of the Esthonian Silurian as determined by Schmidt ! 
are as follows, the highest division being named first. 
Upper Oesel zone, stage K or 8, thickness, 50-75 ft. 
Lower Oesel zone, stage I or 7, thickness, 100 ft. 
Pentamerus estonus zone, stage H or 6, thickness, 50 ft. 
Raikull beds | substage G3 or 5, thickness, 20 ft. 
Borealis bank ; stage G ‘i G2 or 4, thickness, 10 ft. 
Jérden beds J © G1 thickness, 20-30 ft. 
For these names the following are proposed : — 
Oesel formation for stage K. 
St. Johannis formation for stage I. 
Addifer formation for stage H. 
Tamsal formation for stage G. 
There are several reasons why stage K. should be subdivided; but, 
as the outcrops are hardly sufficient to determine the exact sequence 
and the relation of the upper subdivision to the lower, it is considered 
wiser to leave the formation undivided. ‘The matter will be further 
considered under the proper heading. 
Tamsal formation. The three lowermost divisions of the Silurian 
were grouped together by Schmidt, and there appear to be no good 
reasons for changing the grouping. Where well defined, the Borealis 
Banks are worthy of distinction; but as these shells appear to enter 
into both the strata above and below, it would appear unwise to make 
a formational separation from either. At no place can the three 
divisions be seen together. 
(a) Jorden zone. The Jérden zone makes its most eastward ap- 
pearance just west of Lake Peipus and thence can be traced to the 
sea-coast a short distance south of Hapsal. Of the islands, Dago 
appears to be the only one on which it occurs. Since no section is 
known showing the contact with both the Borkholm and the succeed- 
ing Borealis banks, its thickness is quite uncertain. Schmidt? 
estimated that it may lie between twenty and thirty feet. 
1Schmidt. Quar. journ. Geol. soc. London, 1882, p. 525. 
2Schmidt. Loc. cit., 1882, p. 526. 
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