TEENTON 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. 



{Stale Collection ; Cabinet of Dr. Chawe.) 



74. 7. PLEUROTOMARIA UMBILICATA. 



Pl. XXXVIII. Figs. 1 a, b, c, d, e,f, g. 

 Reference Pleiurotomaria uminlioata, pag. 43, pi. 10, figt 9 of this volume. 



This Tariable species is widely distributed in the Trenton Hmestone, 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 o'f 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 ia apparently identical with the New- York specimens, 

 though the spire appears to be more elevated than in the prevailing forms. 



Mr. Conrad {Proc. Jicad. JVat. Sci. Philadelphia, Vol. i. p. 330) regards this shell as 

 identical with Pleurotomaria angulata of Sowerby {SU. System, pag. 641, pl. 21, f. 20); 

 but in this species, the cast of the volutions is distinctly biangular, thus differing from the 

 figare of Sowerby. 



Fig. 1 a. Back of the shell, showing an elevated spire. 



Fig. 1 b. Front view, showing the aperture. 



Fig. 1 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 / 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 shaly 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. (-''''"« Collection.) 



