228 
PALAEONTOLOGY OF NEW YORK. 
calcareous sandstone to an arenaceous limestone; and gradually passes upward 
into the limestone of the Upper Helderberg group, of which it forms an 
important member. With the passage of this rock into a limestone, the occur¬ 
rence of Orthoceratites almost entirely ceases,—at least, in the eastern part of 
the State. 
Notwithstanding the number of species, and the great number of individuals, 
a very small proportion of the whole preserve the surface-markings. They 
are almost invariably in the condition of casts of the interior, the shell hav¬ 
ing been dissolved by the percolation of water through the coarse material of 
the rock. In some examples, where the rock is less charged with arenaceous 
matter, the matrix adheres so closely, seemingly cemented to the fossil, that 
no satisfactory evidence of surface-markings can be obtained. It is rarely 
possible to determine the character or thickness of the exterior shell of the 
Orthoceratites in the Schoharie grit. The septa are extremely thin, often 
broken or distorted, through the process of filling with sediment, while the 
external form of the shell is preserved. The siphuncle, though usually well 
marked in its passage through the septa, is rarely to be found in the interme¬ 
diate space, and, in the best examples, is only partially preserved. Specimens 
which have been cut longitudinally directly through the siphuncle, as shown on 
the septa at the two extremities, preserve no evidence of that organ in its 
passage through the chambers, and only a simple mark or notch in the interme¬ 
diate septa. We can account for this absence only upon the supposition that 
the tube has been so thin that its walls have been dissolved or broken away dur¬ 
ing the process of filling the cavity with the surrounding sediment. In many 
cases it appears as if the siphuncular tube may have been absorbed or otherwise 
removed before the filling of the cavity began. Numerous specimens have 
been cut for the purpose of determining the presence and character of the 
siphuncle, but, in by far the greater number, without any satisfactory result. 
In addition to these difficulties, the fact that the specimens are nearly all 
fragmentary, much compressed and often distorted, will render it evident that 
the study of these fossils must be in the last degree difficult, and the final 
result unsatisfactory. 
The plates illustrating the species of Orthoceratites from the Schoharie 
