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 Europoe. 
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 Brjmzoa 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- 
