40« PALAEONTOLOGY OF NEW-YORK. 



some parts of Virginin, that in the friable sandstone the shells are en- 

 tirely silioified and quite free from adhering stone, so that the exterior 

 markings and internal structure are perfectly preserved ; the interior 

 being quite hollow, or filled only with loose sand. In these localities, not 

 only do we find the cavities of large gasteropods with no more adhering 

 matter than those of the Tertiary sands, but not unfrequently the delicate 

 internal apparatus of the Brachiopoda is almost entirely preserved ; fur- 

 nishing means for the determination of several genera, which the collec- 

 tions from all other parts of the country have failed to show. 



The grtat changes in the physical conditions supervening at the close 

 of the preceding group indicate an influence which would affect in an 

 equal manner the fauna of the succeeding one, and we find accordingly 

 few species passing from the Lower Helderberg group to the Oriskany 

 sandstone. The changes, however, are mainly of a specific character; no 

 new genera being introduced, so far as we now know, though some of them 

 appear under modified forms. 



Amongst the Brachiopoda, Orthis, Strophodonta, Chonetes, Strophomena 

 and Leptana, Spirifer, Cyrtia, Rhynchonella and Merista are genera which 

 occur equally in this formation and in the preceding and succeeding ones. 

 At the same time the Genera RensselcRria, Eatonia and Leptocalia are 

 more fulh' developed than at any former or succeeding epoch. Of the 

 Genus Leptoccelia we have one species in the Niagara group and two in 

 the Lower Helderberg group, two of larger size in the Oriskany sand- 

 stone, and one or two in the succeeding rocks. Of the Genus Eatonia we 

 have four species in the rocks of the Lower Helderberg group ; and we 

 do not at present know more than an equal number in the Oriskany 

 sandstone, though they are more abundant in the latter rock, and one 

 species is larger than the largest of the Lower Helderberg species. Of the 

 remarkable form Rensselaeria we have four species in the rocks of the 

 Lower Helderberg group, while the genus becomes excessively developed 

 in the Oriskany period ; not so much, however, in the number of species, 

 as in their large size and the great number of individuals. We already 

 know throe species of this genus in rocks above the Oriskany sandstone. 



