228 PALEONTOLOGY 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 siphunoular 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 



