98 BEACHIOPODS. 



Zygospira modesta (SAY). 



Atrypa modesta Say, 1847: Pal. New York, vol. 1, p. 141, pi. xxxiii, 



fig. 15. 

 Zygospira modesta Hall, 1862: 15th Rep. Rpg. New York State Cab. 



Nat. Hist., p. 154. 

 Zygospira modesta Meek, 1873: Geol. Sur. Ohio, Pal., vol. I, p. 125. pi. 



xi, figs. 4a-d. 



Shell small, rather depressed, nearly plano-convex, sub- 

 orbicular, or, sometimes, a little wider than long; posterior 

 lateral margins often slightly straightened and converging to 

 the beaks at an obtuse angle; lateral margins more or less 

 rounded ; front rounded, or, sometimes, a little straightened, 

 or very slightly sinuous at the middle. Dorsal valve with a 

 rather shallow, undefined mesial sinus of moderate breadth at 

 the front, but becoming rapidly narrower, and less impressed 

 posteriorly, so as often to die out before reaching the umbo ; 

 surface on each side of the sinus gently convex centrally, 

 and sloping to the lateral margins; beak but slightly prominent 

 and incurved. Ventral valve, with a low mesial ridge, corre- 

 sponding to the sinus of the other valve, excepting that it is 

 generally most prominent near the middle, and somewhat de- 

 pressed anteriorly ; while on each side of the ridge the slopes 

 are distinctly compressed ; beak small, abruptly pointed, pro- 

 jecting beyond that of the other valve, and rather distinctly 

 arched ; but not so closely incurved as to conceal the small 

 fissure, which seems to be closed below by a deltidium, that 

 leaves a minute aperture above, just under, or extending to, the 

 apex; margin on each side of beak carinated, so as to give the 

 appearance of a kind of false cardinal area. Surface of each 

 valve ornamented by about 16 to 18 small, simple, radiating 

 plications, of which about three to five near the front of the 

 dorsal valve occupy the mesial sinus, the middle one being 

 usually a little the largest ; while on the ventral valve about 

 four of the largest occupy the mesial prominence the furrow 

 between the middle two being generally a little larger and 

 deeper than the others ; marks of growth undefined, or ex- 

 tremely minute and obscure. ( Meek.) 



