318 Viscount d'ARCHiAc and M. de Verneuil 



that the Calcaire a Producte is blended with the coal-measures. The genus is still 

 to be seen above the coal-measures, but disappears after the magnesian limestone 

 or the zechstein, and the corresponding beds in the governments of Archangel, 

 Vologda, Perm and Orenbourg*. 



The LeptancE, among which we comprehend the greater part of the Strophomence, 

 present, according to our views, characters which place them naturally between the 

 Producti and the Orthides ; but as relates to their distribution among the three 

 systems, it is analogous to that of the species of this latter genus, for there are twenty- 

 two Silurian, ten Devonian, and four Carboniferous, The Silurian species, among 

 which we may particularly distinguish the L. sericea (Sow. Sil. Syst.) as one that 

 is found in the most distant countries, are principally from the border counties of 

 England and Wales, and from the grauwacke of the banks of the Rhine. Some 

 are found in Scandinavia and in the dependent isles, also in the state of New 

 York f . The Leptcena sarcinulata (L. lata of M. de Buch), the most common 

 species in Europe, and which passes into the Carboniferous system, has been men- 

 tioned, but with a doubt, as occurring in the south of Africa J. Of ten Devonian 

 species, one, the L. analoga, Phill., is also found in the superior system ; it is one 

 of the most common not only in England but in Belgium, and in the states of 

 Ohio and New York. 



The genus Orthis^, which is much more numerous than the genus Leptcena, fol- 

 lows, in its vertical distribution, a precisely analogous ratio of decrease. Sixty-four 

 species are Silurian, thirty-five Devonian, and fifteen Carboniferous. Nine species, 

 common to the first two systems, serve to denote an intimate connexion between 

 them, such as the Polyparia have already demonstrated. The 0. Callactis is the 

 most exclusively Silurian species, and is found in England, in Scandinavia, in 

 Russia, in the state of New York, and in the south of Africa. The 0. lunata 

 (Sow. in Sil. Syst.), and 0. testudinaria, Dalm.|l, which also pass upwards into the 

 Devonian system, are frequent in various parts of Europe and North America. 



* We are happy to inform the palaeontologist that M. de Buch has just completed a monograph of the 

 genus Productus, which will complete his works upon the Brachiopoda family, on which he has tlirown 

 so much light. {June IS^S.) 



f We have seen in the interesting collection of Mr. Logan, who is well acquainted with the geology 

 of some parts of North America, that the Leptana sericea is one of the most common shells at Montreal 

 and in the limestone of Upper Canada. {May ISiS.) 



X Silur. Syst., p. 583, note, 



§ We have united several of M. Pander's genera to Orthis, and have much reduced the number of 

 species which he had established provisionally. 



II Orthis lunata, O. testudinaria, O. hyhrida, O. basalis and O. elegantula, characterized by their 

 bent striae, the first of which turn back to the hinge, are difficult to distinguish from each other by the 

 insufficient descriptions which are usually given.. 



