PLATE LI. 
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Fenestella (Unitrypa) acaulis. 
Page 131. 
1. The celluliferous face of a specimen, natural size, showing the summits of the carinse and scalse. 
2. A portion of the non-celluliferous face of the preceding. (X 6.) 
3. A portion of the non-celluliferous face of another specimen, showing more slender branches. 
(X 6.) 
4. A lateral view of a branch, showing the cell apertures, minute pits in the base of the carina, and 
a transverse section of the lamelliforin connecting bars, or scalse. 
5. The celluliferons side of a fragment, from which the carinse and seal® have been removed, show¬ 
ing two rows of cell apertures on each branch. (X 6.) 
6. An enlargement, to six diameters, of fig. 1, showing more distinctly the character of the summits 
of the carinse and scalse. 
Upper Helderberg group. ' Falls of the Ohio river. 
Fenestella (Unitrypa) stipata. 
Page 134. 
7. The non-celluliferous face of a fragment, natural size. 
8. A portion of the preceding, showing the ang-ular branches and irregular fenestrules. (X 6.) 
9. The non-celluliferous face of another fragment, natural size. 
10. A portion of the same, showing smaller fenestrules than the preceding specimen. (X 6.) 
11. The celluliferous face of a fragment from which the carinse and scalse have been removed. The 
branch at the left of the figure shows the natural condition of the cell apertures ; on the two 
adjacent branches the margins of the apertures are thickened and the openings closed ; the 
next branch shows the margins broken away, and in the right hand branch the entire upper 
test has been removed, showing the interior of the cells. 
12. A fragment, natural size, showing the summits of the carinse and scalse. 
13. A portion of the same. (X 6.) 
14. A transverse section, showing the form of the branches, the height of the carinse and the extent 
of the scalse. 
Upper Helderberg group. Falls of the Ohio river. 
Fenestella (Unitrypa) tegulata. 
Page 135. 
15. A portion of the non-celluliferous face of a specimen. (X 6.) 
16. The non-celluliferous face of another specimen, showing a marked contrast with the preceding, 
in its irregular angular branches and fenestrules. (X 6.) 
17. The celluliferous face of fig. 15, with the carinse and processes removed; the right-hand branch 
has the tissue broken away, showing the interior. (X 6.) 
18. The celluliferous face of the same, natural size, showing summits of the carinse and scalse. 
19. A portion of the same, enlarged, showing more distinctly the characters of the specimen, fig. 18. 
(X 6.) 
20. The summits of the carinse with the scalse, from the celluliferous face of fig. 16. (X 6.) 
21. A transverse section, from the specimen fig. 15, showing the form of the branches, the elevation 
of the carinse and the extent of the scalse. (X 6.) 
22. A lateral view of a branch, natural size. 
23. A portion of the same enlarged, showing the direction of the lamelliforin scalse. (X 6.) 
Upper Helderberg group. Falls of the Ohio river. 
