274 SYSTEMATIC PALEONTOLOGY 



a size beyond the largest figures given. The valves are often close pressed, 

 and deflected at the margin." Hall, 1847. 



Occurrence. CHAMBERSBURG LIMESTONE (Christiania bed). Green- 

 castle and other localities in southern Pennsylvania. Trenton limestone 

 of New York. 



Collection. U. S. National Museum. 



ZYGOSPIRA MODESTA (Hall) 

 Plate LIV, Figs. 20-22; Plate LVII, Figs. 13-16 



Atrypa modesta (Say) Hall, 1847, Pal. New York, vol. i, p. 141, pi. xv, fig. 15. 

 Zygospira modesta Meek, 1873, Pal. Ohio, vol. i, p. 125, pi. ii, fig. 4. 

 Zygospira modesta Winchell and Schuchert, 1893, Geol. Minnesota, vol. iii, 



p. 465, pi. xxxiv, figs. 42-44. 

 Zygospira modesta Hall and Clarke, 1893, Pal. New York, vol. viii, pt. 2, 



p. 155, figs. 146-149; pi. liv, figs. 7-10, 12. 



Description. " Shell small, rather depressed, nearly plano-convex, sub- 

 orbicular, or 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 gradually to the 

 lateral margins ; beak but slightly prominent and incurved. 



" Ventral valve, with a low mesial ridge, corresponding to the sinus 

 of the other valve, excepting that it is generally most prominent near the 

 middle, and somewhat depressed anteriorly; while on each side of the 

 ridge the slopes are distinctly compressed ; beak small, abruptly pointed, 

 projecting 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 cari- 

 nated, so as to give the appearance of a kind of false cardinal area. 



