TRENTON LIMESTONE. 175 
These figures are of specimens of the ordinary size; they are frequently smaller, and 
others attain an elevation of nearly two inches. 
Position and locality. This species occurs in the lower part of the Trenton limestone, near 
its junction with the Black-river mass, at Watertown, Jefferson county. I have also seen 
the same species in the shales of the Hudson-river group at Turin and Pulaski. 
(State Collection ; Cabinet of Dr. Crawe.) 
74. 7. PLEUROTOMARIA UMBILICATA. 
Pl. XXXVIII. Figs. 1 a, b, c, d, e,f, g. 
Reference Pleurotomaria umbilicata, pag. 43, pi. 10, fig. 9 of this volume. 
This variable species is widely distributed in the Trenton limestone, occurring in nearly 
all localities of the rock. The description of the forms in the Birdseye limestone apply to 
those of the Trenton limestone, though they are usually better preserved in the latter rock. 
The last volution is sometimes angulated along the margin of the umbilicus ; and, in a 
single specimen, apparently not differing specifically, there is an additional carina on the 
side of the shell. If this prove only a deviation from the prevailing character of the shell, 
we may be inclined to refer the P. quadricarinata (page 43 of this Report) to the same 
species. The specimens, in both instances, however, are much compressed and partially 
distorted, so that further examination may be required to determine this point. 
The specimen fig. 8 g is from Mineral Point ( Wisconsin), having the upper part of the 
spire replaced by sulphuret of lead. It is apparently identical with the New-York specimens, 
though the spire appears to be more elevated than in the prevailing forms. 
Mr. Conrad ( Proc. Acad. Nat. Sci. Philadelphia , Vol. i. p. 330) regards this shell as 
identical with Pleurotomaria angulata of Sowerby ( Sil. System , pag. 641, pi. 21, f. 20); 
but in this species, the cast of the volutions is distinctly biangular, thus differing from the 
figure of Sowerby. 
Fig. 1 a. Back of the shell, showing an elevated spire. 
Fig. 1 b. Front view, showing the aperture. 
Fig. I c. Base of the shell, showing the umbilicus. 
Fig. 1 d. View of the top of the spire of a larger specimen. 
Fig. 1 e. Front view of the same, showing a depressed spire. 
Fig. 1 f Base of the same, showing the large umbilicus. 
Fig. 1 g. Specimen from Mineral Point, showing a greater elevation of the spire. 
Position and locality. This species occurs in almost every part of the Trenton limestone, 
usually in the intercalated slialy strata, and more rarely in the compact or crystalline 
portions of the rock. It has been found at Middleville, Herkimer, Trenton Falls, Lowville, 
Turin, Watertown and Adams, and probably occurs in many other localities. 
This species has already been shown to exist in the Birdseye limestone, and it has been 
rarely seen in the Hudson-river group. (State Collection.) 
