PLATE XLI— Continued.] 



Meristina UECTIROSTRA, Hull. 



PijT. 18. A young shell with elongate valves. 



Fig-B. 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. Waldron, Indiana. 



Meristina Blancha, Billings. 



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



Lower Helderberg. Square Lake, Maine. 



Meristina tumida, Dalmiui. 



Fig. 4. An internal cast ; showing the median septum, muscular scars ami vascular sinuses of the brachial 

 valve. 



Wenlock limestone. Wenlock Edge, England. 

 Figs 24, 2,'). Two views of a specimen rather below the average size ; showing the low, concave median 

 told on the brachial valve. 



Upper Silurian. Westergam, Island of Ootland. 



Genus HINDELLA. Davidson. 



I'agti 03. 



HiNDELLA UMBONATA, Billillgri. 



Figs. 26, 27. Two views of a sjvecimen 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 umbones on both valves. 

 Middle Silurian. Junctiun Cliff, Anticosti. 



Hindella Prinstana, Billings. 



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 B, umbonata, represented in figs. 29 and 30. li; a large 

 number of these shells it is very difficult to fix upon determinative characters ilistinguishing 

 these two foims. 



Middle Silurian. Junction Cliff, Anticosti. 



Genus KAYSERIA, Davidson. 



Page 101. 



Kay.'ieuia 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 specimen ; showing the relatively slight convexity of the valves. X 2. 

 Middle Devonian. Eifel, Germany. 



