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 Ijecause 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, ^Yassalem 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 Camlirian-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 thev are treated onlv incidentallv 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 



