PLATE XLI—Continued.] 
Meristina rectikostra, Hall, 
Fig". 18. A young- shell with elongate valves. 
Figs. 19, 20. The mature shell; showing its usual outline and the erect beak of the iiedicle-valve. 
Fig. 21. The beaks of an adult specimen, enlarged to show the unclosed triangular delthyrium. X 3. 
Niagai-a group. Waldron, Indiana. 
Meristina Blancha, Billings. 
Figs. 22, 23. Two views of the original specimen. 
Lower Helderberg. Square Lake, Maine. 
Meristina tumida, Dalman. 
Fig. 4. An internal cast; showing the median septum, muscular scars and vascular sinuses of the brachial 
valve. 
Wenlock limestone. Wenlock Edge, England. 
Figs 24, 25. Two views of a specimen rather below the average size; showing the low, concave median 
fold on the brachial valve. 
Upper Silurian. Westergarn, Inland of Gotland. 
Genus HINDELLA, Davidson. 
Page G3. 
Hindella umbonata, Billings. 
Figs. 26, 27. Two views of a specimen which is extreme in the degree of contraction of the umbo of the 
pedicle-valve. X 2. 
Figs. 29, 30. The moi-e usual form of the shell, having full, shouldered umbones on both valves. 
Middle Silurian. Junction Cliff, Anticosti. 
Hindella Prinstana, Billings. 
Fia-. 28. The usual form of this shell, which is more orbicular in outline and less distinctly angulated at 
the cardinal angles than in the tyj^ical H, umhonata, represented in figs. 29 and 30. In a large 
number of these shells it is very difficult to fix upon determinative characters distinguishing 
these two foi-ms. 
Middle Silurian. Junction Cliff, Anticosti. 
Genus KAYSEEIA, Davidson, 
Page 101. 
Kaysekia lens, Phillip.s. 
Fig. 31. Dorsal view of an average example; showing the slight projection of the beak of the pedicle- 
valve, and the fine plication of the median sinus. X 2. 
Fig. 32. Profile of the same specimen ; showing the relatively slight convexity of the valves. X 2. 
Middle Devonian. Eifel, Geinnany. 
