CEPHALOPODA. 231 



The vertical distribution of the species of Orthoceratites, as indicated in the 

 preceding table, offers some facts of general interest. 



The Potsdam sandstone, with its wide geographical range in the United 

 States and Canada, has not afforded a single species of Orthoceras, although 

 prolific in its Trilobitic fauna, and its Linguloid and Oboloid forms, which are 

 known in numerous localities over an extent of more than two thousand miles. 

 In these respects the formation corresponds to the primordial zone of Europe. 

 In the Calciferous, Quebec and Chazy groups we have fifty-three species of 

 Orthoceras and three of Endoceras; of which, only three species pass into any 

 succeeding formation. In the Black-river and Trenton groups we have sixty- 

 one species of Orthoceras and twenty-two of Endoceras ; of which, only four 

 pass into succeeding formations. The Hudson-river group, including the Utica 

 slate and Anticosti group, contains thirty-five recorded species of Orthoceras 

 and two of Endoceras ; of these, two species pass upward from the Trenton 

 limestone. While there are the most intimate relations between the fauna of 

 the Trenton and Hudson-river groups in all other classes, the Cephalopoda 

 form a marked exception. In the Clinton and Niagara groups we have sixty- 

 two species of Orthoceras recorded. In the Lower Helderberg group we have 

 but nine species recorded, although a few other forms are known to occur in 

 that horizon. This is in most striking contrast with the Niagara group, while 

 the fauna in the other classes strikingly correspond. 



In the character and abundance of the Brachiopoda these two formations are 

 almost identical ; while the Gasteropoda are even more abundant in the Lower 

 Helderberg than in the Niagara group. In the growth and development of the 

 Corals and Bryozoa these formations are extremely similar; and there is a 

 great similarity in the physical conditions of the groups throughout, except in 

 the absence, to a great extent, of the deposition of Magnesian limestone in the 

 Lower Helderberg period. 



In the Upper Helderberg group, of which the total thickness is less than the 

 Lower Helderberg, we have thirty species of Orthoceras recorded ; while the 

 Hamilton group, with a thickness more than five times as great, has yet 

 afforded but twenty-nine species. The Chemung group, with its greater thick- 



