t 



BIBDSETE LIMESTONE. 43 



73. 6. PLEUROTOMARIA QUADRICARINATA (n. *p.). 



Pl. X. Fig. 8. 



Trochiform ; spire obliterated ; last whorl with four distinct carinse ; surface marked with 

 zigzag striae, 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 striae, 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. 5p.). 



Pl. X. Figs. 9 a, b, e, d, e, g, h. 



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, 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 

 firagments, 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 A. Lower side of a small specimen. 



When imbedded in the rock, and often much distorted, the different phases of this fossil 



6* 



-./ 



