PLATE XLI-ConUnaed.] 



Meristina rectirostka, Hall. 



Fig-. 18. A younfi^ shell with elongate valves. 



Figs. 19, 20. The mature shell ; showing its usual outline and the erect beak of the pedicle-valve. 

 Fig. 21. The beaks of an adult specimen, enlarged to show the unclosed triangular delthyrium. X 3. 

 Niagai-a group. Waldrcm, Indiana. 



Meristina Blancha, Billings. 



Pigs. 22, 23. Two views of the original specimen. 



Lower Helderberg. Square Lake, Maine. 



Meristina tcmida, 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, 26. Two views of a specimen rather below the average size ; showing the low, concave median 



fold on the brachial valve. 



Upper Silurian. Westergam, Island of QoUand. 



Genus HINDELLA, Davidson. 



Page 61. 



HiNDELLA UMBONATA, Billings. 



Figs. 26, 27. Two views of a sjjecimen which is extreme in the degree of contraction of the umbo of the 



pedicle-valve. X 2. 

 Figs. 29, 30. The more usual form of the shell, having full, shouldered nmbones on both valves. 

 Middle Silurian. Junction Cliff, Anticosti. 



HiNDELLA Prinstana, Billiugs. 



Fig. 28. The usual form of this shell, which is more orbicular in outline and less distinctly angulated at 

 the cardinal angles than in the typical H. uinbonata, represented in figs. 29 and 30. In a large 

 number of these shells it is very difficult to fix upon determinative charactei-a distinguishing 

 these two foi-ms. 



Middle Silurian. Junction Cliff, AjUieosti. 



Genus KAYSERIA, Davidson. 



Page 101. 



Kayseria lens, Phillips. 



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 si>ecjmen ; showing the relatively slight convexity of the valves. X 2. 

 Middle Devonian. E\fel, Oermany. 



