BLACK-RIVER LIMESTONE. 57 



contact with the external shell. Fragments like this one are of frequent occurrence, inducing 

 a belief that it is a distinct species ; and the figure is given here for the purpose of explaining 

 the manner in which such a section is produced, which may be either ty a previous bending 

 of the shell, or from wearing down unevenly upon the surface. 



Fig. 1 c. A longitudinal section, similar to the last, but the shell not bent ; the direction of the section is 

 nearly in the ventral and dorsal linea Septa and outer shell showing double laminae. 



Fig. I d. A longitudinal section, showing the siphuncle apparently central, which is due to a wearing 

 down from the ventral side, and consequently leaving the siphon equidistant from the two 

 lateral margins of the shell. 



Fig. 1 «. A similar section to the last, both of which exhibit more or less distinctly the double laminae 

 of the outer shell and septa. 



In figs. 1 b, c, e, the double laminae of the outer shell and septa are not so clearly dis- 

 tinguishable, the interspaces in the fossil being filled up by calcareous matter, so that both 

 shell and septa appear thickened. In 1 a and 1 d, particularly the latter, the double laminae 

 of the septa are quite distinctly preserved, the interspaces being empty. This structure will 

 be more fully illustrated when treating of the generic character of the Orthocerata. 



Platb XVII. 



Fig. 1. A fragment of the same species, showing the siphon directly in contact with the ventral side of 

 the shell, a portion of which remains on the lower part of the specimen. There is also a 

 slight irregularity in the size of the annulations, which is a common occurrence. 



Fig. 1 a. Transverse section of the last, showing the wrinkled inner surface of the siphon at the points 

 of contraction.* 



Fig. 1 J. A longitudinal section, passing through the siphuncle : the interior is here filled with stony 

 matter ; but in other specimens, the inner surface of the tube presents the same wrinkled 

 appearance along the contracted portions as is represented above, fig. 1 a. 



In presenting so many figures of this species, the object has been to prevent misapprehen- 

 sion, and to present to the student in the science the most obvious features, and those 

 apparent variations which he might seize upon to form distinct species. Doubtless they will 

 be presented under other aspects ; but it is believed that a careful study and comparison of 

 these figures may enable one to detect the important characteristics, and decide their identity 

 with these forms. 



In the Transactions of the Geological Society of London, Second series, Vol. i. pi. 30, are 

 to be found some analogous forms ; and if there be no fallacy in these, which I somewhat 

 suspect, there are several species indicated. From the geographical position of these fossils, 

 it may be inferred that their geological position is identical with those of New- York. 



Position and locality. In the Black-river limestone, associated with Gonioceras, Lituites, 

 Columnaria, &c. &.c. Watertown and Henderson Bay, Jefferson county ; Copenhagen and 

 Lowville, Lewis county ; rarely in the Mohawk Valley, Lake Huron, &c. (.State Collection.) 



• See figures of AcTWOcEaAs in Tyan$. Geol. Soc. London, 2d series. Vol. v. pi. 19. 

 [Pal.«:ontolooy.] " 8 



