104' GEOLOGICAL MEMOIRS. 



bourhood of St. Petersburg and the cliffs of Esthonia are celebrated 

 for the abundance of their species, the whole number of which in 

 Russia amounts to twenty-three, of which fourteen are Silurian, 

 three Devonian, one Devonian and carboniferous, four carbonife- 

 rous, and one Permian. This number is not more than one-third 

 of the whole number of species quoted by various authors, but many 

 of those described will no doubt require to be suppressed ; it forms 

 about two-thirds of the species distinctly made out. The small spe- 

 cies with simple folds are as usual characteristic of the more ancient 

 beds; those found in the carboniferous and Permian rocks are larger, 

 and belong to our division Recto- striatcBy having a single area, and 

 they may readily be distinguished from the other species of Orthis 

 usually provided with a double area. The most common Russian 

 species are O. arachnoidea, which is nearly allied to O. pecten, but 

 is much higher in stratigraphical position ; O. caligramma and O. 

 parva, which either have representatives or are actually repeated by 

 identical species in Sweden and America; and O.resupinata, which, 

 as well as the variety striatula, is widely extended vertically as well 

 as horizontally, passing from the Devonian into the carboniferous 

 system, and found in the Altai Mountains and in the Ural, in the 

 plains of Russia and Poland, in Belgium, in England, in the Astu- 

 rias, and lastly, on the other side of the Atlantic in North America, 

 where it has been mentioned by Mr. J. Hall as occurring in the 

 state of New York. The O. crenistria is in Russia as in America 

 a Devonian species, while in England it ranks as carboniferous. 



Leptcena, considered, not as synonymous with Productus, but as a 

 distinct genus resembling Orthis, from which it is distinguished by 

 the concave ventral valve, is represented in Russia by fourteen species 

 out of the twenty-four or twenty-five at present known. One only 

 of these, L. Ouralensis^ belongs to t\yo systems, and not one occurs 

 so high as the carboniferous series, all of them being confined to 

 the two lower divisions of the palaeozoic group. The decrease in 

 the case of these two is so marked (falling at once from eleven to 

 four species), that we might have expected, a priori, their early ex- 

 tinction. Among the most interesting species we may mention L. 

 sericea, which occurs not only in Russia, Sweden and England, but 

 also on the Asiatic shores of the Thracian Bosphorus and in North 

 America; L. imbrex, which Captain Bayfield has found in Canada ; 

 and lastly, L. Dutretii, which M. Paillette has just discovered in the 

 schists and Devonian limestones of the Asturias. The species L. 

 depressa, which in Russia is exclusively Silurian, in other countries 

 passes into the Devonian system, and following the law that those 

 species which extend through different systems are also widely dis- 

 tributed in space, this one is met with in a vast number of locali- 

 ties, and has become one of the most common fossils of the double 

 period. 



The little genus Chonetes *, only comprising two species in Rus- 



* The species of Chonetes recently discovered in America by Mr. J. Hall prove, 

 by the persistence of the characters which we have assigned to this genus, how 

 well-founded it is in nature. 



