DE VERNEUIL AND d'aRCHIAC ON RUSSIAN PALiEOZOIC FOSSILS. 107 



diacese the groups of dimyarian Acephala which most abound in the 

 palaeozoic rocks, taken as a whole; and reckoning up all the species 

 described by various authors, we find the number of the first (Nu- 

 cula) to be thirty-seven, and that of the second genus (Area) seven- 

 teen, from which small number it appears that the physical condi- 

 tions in Russia could not have been very favourable for their de- 

 velopment ; when however we remember that the species of these 

 genera generally have a local and very circumscribed development, 

 it will not seem impossible that future discoveries may raise this 

 number, and render it proportionably as great as that which cha- 

 racterises other countries. 



The genus Megalodon, which is exclusively Devonian, is repre- 

 sented by only one or two species in Russia. Of Mytilus (including 

 Modiola) there are, on the other hand, six species recognized by 

 ourselves, besides two that have been named by M. Eichwald, and 

 of these M, Pallasi is the most widely spread, and it is also one of 

 the most characteristic fossils of the Permian system. The Pinna 

 hardly seems to have shown itself during the palaeozoic period, but 

 was represented by two species smaller than those of our own time, 

 one of them occurring in the coal-shales of Lissitchia-Balka (Do- 

 netz), while the other is met with in the English mountain lime- 

 stone. 



Among the Acephala monomyaria only two genera, Avicula and 

 Pecfen, seem to have been greatly developed. Of the first-named 

 there are however nine or ten species, of which six are peculiar to 

 Russia. 



Considered in regard to their development in time, the Russian 

 species are thus distributed : — two are Devonian, two carboniferous, 

 four Permian, and two occur in the Muschelkalk of Mount Bogdo, 

 Among the latter is the A. Alberti of the German trias, and of the 

 whole number only one, A. antiqua^ ofi'ers the rare example of a 

 species surviving through several systems. The A, subpapyracea 

 of the coal-shales of Lissitchia-Balka greatly reminds one o\ A.pa- 

 pyracea from the similar deposits in England and Belgium, and A, 

 Kazanensis of the Permian system affords a striking analogy in the 

 great inequality of its valves with the A. grijphoidesy which lived at 

 the same time in the English seas. Although however the Aviculas 

 are very rare in the Silurian system in Russia, such is not the case 

 in North America, where we meet with upwards of fifteen of the 

 twenty-two species found in the deposits of this age. 



Pecten was introduced somewhat later, and is totally unknown in 

 the Silurian rocks of Russia as elsewhere. It is however represented 

 by one Devonian species, but only appears abundantly in the car- 

 boniferous system, where we can already enumerate five species. 

 This increase is analogous to that which has been found to take 

 place in other countries; for, in Ireland, Mr. Griffith and Mr. M^C'oy 

 have discovered no less than fifty-six new species in the carbonife- 

 rous limestone, bringing up the whole number of palieozoic species 

 of Pecten to seventy*. 

 * The collections from Van Dicraen's Land, brought by M. Lcguillon and de- 



