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BIRDSEYE LIMESTONE. 43 
73. 6. PLEUROTOM ARIA QUADRIC ARIN AT A (n. sp.). 
Pl. X. Fig. 8. 
Trochiform ; spire obliterated ; last whorl with four distinct carinse ; surface marked with 
zigzag strise, which advance and retreat in passing over the angles of the whorl; aperture 
nearly circular ; umbilicus large. 
It is impossible to characterize the whole of the shell, since all the specimens yet seen 
are more or less imperfect. The angles upon the last volution, with the zigzag striee, seem 
sufficiently characteristic of the shell to denote its specific distinction. 
Position and locality. In the higher shaly part of the Birdseye limestone, associated with 
Phytopsis and the shells which follow. 
74. 7. PLEUROTOMARIA UMBILICATA ( n. sp.). 
Pl. X. Figs. 9 a, b, c, d, e, g, b. 
Depressed, nearly discoidal ; spire gradually ascending; width about equal to twice the 
height; spire short; volutions about four, angular and compressed above, ventricose below; 
suture canaliculated ; aperture subrhomboidal, t angulated anteriorly ; umbilicus large and 
deep ; surface marked with undulating striae. 
The last whorl is distinctly marked by three spiral ridges or carinations : one near the 
suture, one at the upper outer edge, and one at the lower outer edge, leaving the side of 
the volution plain and vertical; the upper side is angulated by the first carina, and the 
lower side rounded, scarcely ventricose; the lower angle of the last volution is covered by 
the spire, the two upper ones continuing. This fossil is readily distinguished, even in 
fragments, by the distinctly canaliculated suture, elevated upper carina, and the curvilinear 
depression between this and the next angle. 
This species is very common, appearing in the Birdseye limestone, and continuing into 
the Trenton limestone, where it is numerous and widely distributed. It assumes various 
forms, which are partly due to compression, but retains its essential characters here given. 
Fig. 9 a. Lateral view of a cast of this species, the spire partially obliterated. 
Fig. 9 b. View of a larger specimen, showing but a single angle upon the cast, the upper one having 
been obliterated. 
Fig. 9 c. View of the upper side of a smaller specimen. 
Fig. 9 d. Lateral view of a specimen much compressed. 
Fig. 9 e. Lower surface of the same, showing the umbilicus and the striae upon the shell, which is 
partially preserved. 
Fig. 9 g. View of the upper side of a crushed specimen. 
Fig. 9 h. Lower side of a small specimen. 
When imbedded in the rock, and often much distorted, the different phases of this fossil 
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