184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Cambrian and Ordovician were lettered in ascending order A to F, 
and many of the subdivisions designated by numbers as, Cj, Cs, C3 and 
finally some of these subdivisions were further divided by Lamansky, 
as for instance B,,;., By;g and B,,,. The strata also received names 
suggested by their lithological characteristics or faunal contents, as 
for instance, B, was also known as the “Glauconite sand” and B,,, as 
the “Orthoceras limestone” or “Vaginatenkalk.” The divisions 
from A to Cig are to be found in the escarpment, and these together 
received the collective name of the “Glint” but no formation in this 
part of the series has received a separate geographic name. 
The strata above Cig are exposed as has already been explained, 
principally in quarries, and therefore each formation has received 
a name from the locality which has furnished either the best expo- 
sures or the best fossils. Thus C2 is known as the Kuckers, E as the 
Wesenberg, etc. 
In this particular case, both the system of lettering and the system 
of descriptive names is objectionable, and for the sake of uniformity 
the writer suggests a set of geographic names for the older formations 
of the series. The system of lettering fails, because A, and Az; of 
Schmidt prove not to be Cambrian but Ordovician, thus splitting A 
between two great systems. ‘There is likewise a difficulty about D» 
which will appear later. A mixed table of descriptive names, part 
derived from lithological and part from faunal characteristics is never 
satisfactory, and in this case the names seem particularly inapplicable. 
Thus the “ Orthoceras”’ limestone is not by any means the only forma- 
tion in Russia in which Orthoceras is abundant, and the term has not 
the same meaning here as when applied to Ordovician strata in Sweden 
or Norway. Likewise C; is called the “ Echinosphaerites”’ limestone, 
though Echinosphaerites is equally common at some localities in Co, 
C3 and the lower part of Dy. 
DESCRIPTION OF FORMATIONS. 
Cambrian. 
Esthonia formation. A, and part of Ag (Blauer Thon and lower part 
of Ungulitensand) of Schmidt. Lower Cambrian. 
Since it has been so well described by Schmidt (42), Mickwitz (33), 
and Holm (20), the writer paid comparatively little attention to the 
study of the Cambrian, but examined the exposures at Reval, Port 
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