TRENTON LIMESTONE. 197 



257. 1. ONCOCERAS CONSTRICTUM (re. JJ9.). 



Pl. XLI. Fig3. 6 a -/, 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 Cvrtoceras. 

 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 «. A shorter and more ventricose specimen. (The transverse lines on the upper half of the figure, 



indicating septa, are incorrect.) 

 Fig. 6/ The apicial extremity of the fossil. Two or three specimens have been found, with an ap- 

 pendage or termination of this kind. 

 Figs. 7 a, e. 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 I have received a specimen 

 from Mr. Joseph Clarke, of Cincinnati, Ohio. (>"'''«'« Collection.) 



