MARYLAND GEOLOGICAL SURVEY 275 



" 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 

 extremely minute and obscure. Length of a mature, moderately large 

 specimen, 0.26 inch; breadth, 0.30 inch; convexity, 0.15 inch." Meek, 

 1873. 



An abundant Cincinnatian fossil at many localities in the United 

 States and Canada. In Maryland and Pennsylvania the species occurs 

 in the upper part (Eden) of the Martinsburg shale and in the succeeding 

 Fairview formation. 



Occurrence. MARTINSBURG SHALE (Eden Division). Eickard Moun- 

 tain and Fairview Mountain, Washington County, Maryland. 



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



ZYGOSPIRA ? ERRATICA (Hall) 

 Plate LVII, Figs. 17-23 



Orthis f erratica Hall, 1847, Pal. New York, vol. i, p. 288, pi. Ixxix, fig. 5. 

 Catazyga erratica Hall and Clarke, 1893, Pal. New York, vol. viii, pt. 2, 

 p. 158, pi. liv, figs. 17-23. 



Description. " Subhemispherical, orbicular ; dorsal valve very convex, 

 with the mesial portion abruptly elevated, flat above ; ventral valve convex 

 at the sides, depressed in the middle, and considerably elevated in front ; 

 surface marked by fine simple uniform striae." Hall, 1847. 



This species is readily distinguished from Zygospira modesta with 

 which it is associated, by its larger size, greater convexity, and especially 

 by the numerous fine striae. A characteristic fossil of the Pulaski shales 

 of New York and Canada. 



Occurrence. MARTINSBURG SHALE (Fairview division). Tuscarora 

 Mountain, one and one-half miles southeast of McConnellsburg, Penn- 

 sylvania. 



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



