182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The palaeontologists have been particularly active in describing 
the Ordovician fossils of this region, as may be seen by the very long 
list of species given by Bassler, (1). Part of this work, on the trilo- 
bites by Schmidt, (48), bryozoans by Bassler, (1), and cystids by 
Jaeckel, (24), is modern, as are also descriptions of certain groups of 
Ostracoda, Cephalopoda, Brachiopoda, and Gastropoda by various 
writers. The bulk.of the Brachiopoda, Pelecypoda, and corals, still 
await monographic treatment, though some of these groups are now 
in the hands of specialists. 
OBJECT OF THE PRESENT PAPER. 
Although so well known and fully described, there still exist in 
text-books many inaccurate statements about the region under dis- 
cussion and there is no modern general treatment of the whole area. 
For these reasons, and because previous papers are mostly in German 
and Russian, and without illustration, there exists in the minds of 
most American geologists only a very vague idea of the character of 
the Russian deposits, and the writer therefore feels justified in re- 
traversing this old and well-known ground, and hoping to add some- 
thing to what has previously been observed. 
During the seven weeks spent in this area I was able to cross the 
outcrop of the Cambrian and Ordovician on the Sjass, the Walchow, 
and the Lawa at Wassilkowa, all south of Lake Ladoga, at Papowka, 
south of Petrograd, at Narwa, from Ontika south to Jewe, from Port 
Kunda south through Wesenberg and Taps to Borkholm, from 
Reval and Baltishport southwest through Kegel, Wassalem and Lyck- 
holm to Hapsal, and also visited the principal localities on the northern 
half of Dago. I was thus able to see all the principal sections and 
type-localities and crossed the Cambrian-Ordovician belt at right 
angles to the strike at frequent intervals throughout the whole length 
of the outcrop. 
Although the Borkholm and Lyckholm are considered by the writer 
to belong to the Ordovician they are treated only incidentally in this 
paper, but are fully discussed by Dr. Twenhofel (Bull. M. C. Z. 56, 
no. 4). 
\ NATURE OF THE EXPOSURES OF THE STRATA. 
Throughout the whole area underlain by the Ordovician strata the 
country is comparatively flat, and the majority of the hills which do 
