CHAZY LIMESTONE. 35 



61. 5. ORTHOCERAS TENUISEPTUM. 



Pl. VI. Fig. 6. 



Cylindrical, gradually tapering, straight ; surface smooth ; section cylindrical ; septa 

 Tcry thin, gently arched, approximate, about j\ the diameter ; siphuncle small 1 



Only fragments of this shell have been found in this limestone, and therefore all its 

 characters cannot be ascertained. It corresponds in many respects with 0. primigenium of 

 Vanuxem (PI. iii. fig. 11); but the specimens of that fossil attain only a small size, while 

 this one is very large. Another difference will be observed in the concavity of the septa, 

 which are less arched in the specimen under consideration. 



Position and locality. Chazy, Clinton county, in the dark limestone, associated with 

 Maclurea magna. The specimens appear on the surface of the strata, and are all more or 

 less weathered. instate Collection.) 



62. 6. ORTHOCERAS BILINEATUM. 



Pi- VII. Figs. 4, 4 a. 



For description, see Orthocerata of Trenton limestone. 



An examination of this specimen, since the plate was engraved, convinces me that it is 

 identical with 0. hilineatum of the Trenton limestone. The specimen was given me by 

 Dr. Emmons, as coming from the Calciferous sandstone, at a locality two miles east of the 

 city of Albany. An examination of the spot has convinced me that the rock in question is 

 the Trenton limestone, thrust up through the Hudson-river slates. The association of fossils, 

 as well as all other circumstances, prohibit its reference to the Calciferous sandrock. 



63. 7. ORTHOCERAS MONILIFORME. 



Pu VII. Fig. 5. ^ 



Elongated, cylindrical, gradually tapering ; surface smooth, or with only fine striae ; 

 septa very slightly arched, distant about \ the diameter ; siphuncle moniliform, contracted 

 at the point of contact with the septa. 



The siphuncle is but little expanded between the septa, which latter, being distant, give 

 an elongated moniliform appearance. The apparent contraction towards the base is owing 

 to the wearing, in an oblique direction, of the surface of the stone in which it is imbedded. 



In describing many of this family of fossils in the older rocks, it will be necessary to rely 

 upon the internal arrangement of the parts, as these. are often presented in natural sections, 

 while the external surface is obscure or inaccessible. 



Position and locality. Chazy, Clinton county, in limestone, with Maclurea magna. 



