1857.] MXJRCHISON SILURIAN ROCKS OF RUSSIA. 4/ 



mon : Phacops Odini, Eichw., Orthis lynx, Eichw., Leptcena imbrex, 

 Lingula quadrata, and Chcetetes Petropolitanus. 



The upper limit of these Lower Silurian strata is characterized by 

 a profusion of Corals, such as Catenipora, Heliolites, Caninia, Sar- 

 cinula org anon, &c. Accompanying these, there are other fossils 

 peculiar to the band, viz. Proetus brevifrons, Ang., Lichas, n. s., 

 Pleurorhynchus, n. s., Strophomena alternata, Atrypa (Spirigerina) 

 marginalis, and Orbicula, n. s. These two latter, however, are asso- 

 ciated with most of the previously mentioned fossils of the second 

 stage ; so that the whole form one natural division. 



This coralline limestone is surmounted by the most ancient stratum 

 of the group classed as the Upper Silurian of Esthonia ; being cha- 

 racterized chiefly by the presence of its smooth Pentameri. 



A marly limestone forms the basement-layer of No. 3, and con- 

 tains Leperditia marginata, Pentamerus linguifer, Strophomena 

 Pecten, Orthis Davidsoni, Rhynchonella aprinis, Vern., and Calamo- 

 pora aspera, D'Orb. This stratum is covered, throughout the whole 

 length of the zone, by the shelly band with Pentamerus borealis, 

 which occupies the highest ground of the mainland of Esthonia, and 

 forms the watershed of all the tract. The bottom beds, with Leper- 

 ditia, observed at a very few spots to the north of the Pentamerus- 

 zone, reappear to the east, west, and south, in a more developed 

 form. In the last-mentioned direction, they serve as the support of 

 a band containing another species of Pentamerus, the P. Esthonus, 

 Eichw., probably identical with the P. oblongus (see Sil. Syst.) ; but, 

 unlike the P. borealis*, this species does not occupy all the rock. 



The associated characteristic fossils are, Bronteus signatus, Phill., 

 Orthoceras canaliculatum, Sow., and Catenipora escharoides. Other 

 fossils, the chief habitat of which is in the next overlying stage, now 

 appear, and these are Encrinurus punctatus, Calymene Blumenbachii, 

 and Atrypa {Spirigerina) reticularis. 



This stage, as already stated, offers points of connexion with the 

 subjacent No. 2, along its line of junction, but the Corals (Catenipora) 

 of the two bands are specifically separable. 



The stage No. 4, which is developed partly on the continent and 

 partly in the north-eastern portion of the Isle of Oesel, consists of 

 two different rocks, the one a bluish-grey marly limestone, the other 

 a dolomite or magnesian limestone. Its chief fossils are, Proetus 

 concinnus, Lichas Gothlandicus, Ang., L. ornatus, Ang., Bumastus 

 Lindsteineri, Ang., Orthoceras annulatum, Sil. Syst., Rhynchonella 

 Wilsoni (Russian variety: see * Russia in Europe,' vol. ii. p. 88), 

 Athyris tumida, Orthis osiliensis, Schrenck, 0. elegantula, Dalm., 

 Leptcena transversalis, Streptoplasma calicula, &c. This zone also 



* In reference to these detailed views, my friend Count Keyserling is of opinion 

 that the band containing the Pentamerus borealis is virtually included in the zone 

 beneath it ; fallacious appearances in this flat and obscure country having led Prof. 

 Schmidt to believe in the superposition of the one to the other. Count Keyserling 

 also thinks that, in consequence of its fossils, more importance should be attached 

 to the brand-schiefer or inflammable schist, which might be considered a distinct 

 subformation. 



