BRACHIOPODS. 99 



Horizon and localities Lower Silurian, Trenton lime- 

 stone : Saint Louis county ( Hambach ). 



Zygospira subconcava MEEK & WORTHEN 



Zygospira subconcava Meek & Worthen, 1868: Geol. Sur. Illinois, vol. 

 Ill, p. 380, pi. vii, figs. la-d. 



A somewhat smaller and more compressed species than Z. 

 modesta, and having finer radiating lines. 



Horizon and localities Upper Silurian limestone : Bai- 

 ley landing (Perry county). 



Camerella calcifera? BILLINGS. 



Camerella calcifera Billing?, 1861 : Canadian Nat. and Geol. , vol. VI, p. 

 318. 



Horizon and loclities Silurian ? Magnesian limestone : 

 Carter county. 



Rhynchonellacapax (CONRAD). 



Plate xll, figs. 12a-b. 



Atrypacapax Coarad. 1842: Jour. Acad. Nat. Sci., Phila., vol. VIII, p. 



264, pi. xiv, fig. 21. 

 Atrypa increbescens Hall, 1847: Pal. New York, vol. I, p. 146, pi. xxxiii, 



fi?s. 14a-y. 



Rhynchonella capax Billings, 1862- Pal. Foss. Canada, vol. I, p. 142. 

 Rhynchonellacapax Meek, 1873: Geol. Sur. Ohio, Pal., vol. I, p. 123, pi. 



xi, figs. 6a-f. 



Shell attaining about a medium size, varying with age 

 from compressed sub-trigonal to sub-globose, old examples be- 

 ing often more convex than their diameter in any other direc- 

 tion ; posterior lateral margins somewhat straightened and 

 converging to the beaks at about a right angle in young shells, 

 but becoming more rounded in the adult; lateral margins 

 rounding to the front, which is more or less distinctly sinuous, 

 or nearly straight in the middle. 



Dorsal valve generally a little more convex than the other, 

 most prominent in the middle, and rounding abruptly, or slop- 

 ing more gently from the central region in all directions; the 

 more elevated part forming anteriorly a depressed mesial ridge 

 that is nearly flat, and occupied by four plications on top, and 

 rarely continues two-thirds of the way to the strongly incurved 



