TRENTON LIMESTONE. 
197 
257. 1. ONCOCERAS CONSTRICTUM (to. sp.). 
Pl. XLI. Figs. 6a-/, and 7 a, b, c, d. 
Shell curving, ventricose in the middle, abruptly constricted near the aperture, and 
rapidly tapering towards the apex; septa very slightly convex, numerous, approximate, 
slightly undulating and bending upwards on the dorsal margin ; section ovate, with the 
dorsal side narrower and somewhat obtusely angular; siphuncle small, dorsal; surface 
striated transversely. 
The characters here given are those presented by a number of specimens examined, 
though with some little variation, depending on local circumstances, or the nature of the 
strata. I have decided to retain all the various forms as one species, though we may yet be 
able to separate them. 
This fossil has the form of Phragmoceras, which it resembles in the contraction of the 
aperture ; while the position of the siphuncle is different, being dorsal as in Cyrtoceras. 
It appears sufficiently distinct from the latter to constitute a new genus, allied to Phrag¬ 
moceras, and in some respects to Gomphoceras. It is recognized when perfect, not only 
by the contraction of the aperture, but by the ventricose character of the lower part of 
the outer chamber and upper part of the septate portion, below which it is again suddenly 
contracted towards the apex. The base of the shell is often nearly perfect, but the apex is 
usually destroyed, or extends into a curved prolongation like that represented in 6 f, in 
-which there is no evidence of septa. I have seen this kind of termination in two or three 
instances, while all the other specimens have been imperfect like 6 d e. The septa are 
exceedingly thin ; and from the slight convexity, the edges of the chambers in the cast are 
remarkably angular and sharply defined, presenting an appearance very different from the 
Cyrtoceras or Orthoceras of the Trenton limestone, and which is sufficient to distinguish 
the species even in casts of a single chamber. 
Fig. 6 a. View of the ventral side of an imperfect specimen. 
Fig. 6 b. Lateral view of the same, showing the sudden contraction below the outer chamber : the 
aperture is imperfect. 
Fig. 6 c. Section of the lower extremity, showing the small dorsal siphuncle. 
Fig. 6 d. A smaller specimen, showing the same form as the last. 
Fig. 6 e. A shorter and more ventricose specimen. (The transverse lines on the upper half of the figure, 
indicating septa, are incorrect.) 
Fig. 6 f The apicial extremity of the fossil. Two or three specimens have been found, with an an- 
pendage or termination of this kind. 
Figs. 7 a, c. Dorsal and lateral views of a fragment, showing the direction of the septa. 
Figs. 7 b, d. Sections of different specimens, showing the position of the siphuncle, and the narrowing 
of the dorsal side of the shell. 
Position and locality. This species is comparatively rare in all the localities examined. 
It occurs at Middleville, in the lower part of the rock ; while at Watertown a few speci¬ 
mens have been seen in the higher part of the same rock, and 1 have received a specimen 
from Mr. Joseph Clarke, of Cincinnati, Ohio. {State Collection.) 
