MARYLAND GEOLOGICAL SURVEY 273 



rather broad, shallow, rounded median sinus, which corresponds with the 

 fold of the pedicle valve and which reaches nearly to the beak. The 

 dimensions of an average specimen are : Length, 6 mm. ; width, 6 mm. ; 

 thickness, 4 mm." Weller, 1903. 



An abundant species in its various forms, in the Black River and 

 Trenton rocks of New York and Canada and the Ohio and Mississippi 

 valleys. 



Occurrence, CHAMBERSBURG LIMESTONE (Tetradium cellulosum 

 bed). Railroad cut two miles southwest of Marion and at Fort Loudon, 

 Franklin County, Pennsylvania, where the Lowville form occurs in some 

 abundance. 



Collections. Maryland Geological Survey, U. S. National Museum. 



ZYGOSPIRA EXIGUA (Hall) 



Plate XLVIII, Figs. 19, 20 



Atrypa exigua Hall, 1847, Pal. New York, vol. i, p. 141, pi. xxxiii, fig. 6. 

 Protozyga exigua Hall and Clarke, 1893, Pal. New York, vol. viii, pt. 2, 

 p. 149, figs. 137, 138, pi. liv, figs. 47, 48. 



Description. " Plano-convex ; length and breadth about equal ; cardi- 

 nal line considerably extended ; dorsal valve elevated in a ridge along the 

 middle, depressed at the sides, and slightly inflected towards the cardinal 

 extremities; beak small, straight, much extended beyond the cardinal 

 line ; ventral valve considerably shorter than the dorsal, depressed-convex, 

 with a broad depression along the center, reaching half way from the base 

 to the beak; beak small, and close pressed into the foramen beneath the 

 beak of the opposite valve; surface scarcely marked with fine concentric 

 lines, and a few indistinct longitudinal rays near the margin. 



" In the largest specimen which I have seen, there are evidences, under 

 a magnifier, of small radii commencing below the center of the valve. 

 Since, however, they are not perceptible to the naked eye, they are of 

 minor importance, unless it should be found that this is the young of a 

 species which changes with growth. The specimens yet seen, however, 

 are minute, and it may properly be doubted whether the species attains 



