372 Systematic Paleontology 



beak small, closely incurved. Dorsal valve convex, sometimes gibbous, 

 and sloping abruptly to tbe margins ; baving a strong mesial fold begin- 

 ning above the center, and produced in a broad flattened and greatly ele- 

 vated extension. Surface marked by fine radiating striae, whicb, in well- 

 preserved specimens, are crossed by much finer concentric stris : a single 

 central one, and sometimes two or three of the striae upon the mesial 

 sinus, are much stronger than the others ; and there is sometimes an im- 

 pressed line down the center of the dorsal valve." Hall, 1859. 



Length 13 mm. ; width 14 mm. 



Occurrence. — Helderbeug Formation, New Scotland Member. 

 Corriganville, Maryland; Keyser, Cherry Run, West Virginia. Becraft 

 Member. Warren Point, Pennsylvania. 



Collection. — Maryland Geological Survey. 



Eatonia peculiaris (Conrad) 

 Plate LXV, Figs. 33-28 



Atrypa peculiaris Conrad, 1841, Fifth Ann. Rept. N. Y. Geol. Surv., p. 56. 

 Eatonia peculiaris Hall, 1859, Nat. Hist. N. Y., Pal., vol. ill, p. 244, pi. xxxviii, 

 figs. 21-26; p. 436, pi. ci, figs. 2a-26r; pi. cia, figs, la-lh, 1861. 



Descriplion. — " Subtriangular, with obsolete, fine, radiating lines; in- 

 ferior valve flat, concave at base, with a linguiform projection ; superior 

 valve with a convex, mesial elevation at base, where the margins of the 

 valves meet above the basal margin, and are notched or serrate ; submargin 

 of the sides of the flat valve serrated." Conrad, 1841. 



" Shell longitudinally ovate, the proportion of length and breadth 

 variable : cardinal margins sloping abruptly from the beaks. Ventral 

 valve depressed convex in the middle towards the beak, and flattened 

 towards the margins, which are abruptly inflected along the cardinal 

 slopes ; below the middle, extended in a broad, not strongly defined mesial 

 depression, which is prolonged in front into a linguiform extension : beak 

 moderately elevated, perforate. Dorsal valve convex in the middle, and 

 sloping abruptly to the lateral margins ; the central part below the middle 

 elevated into a rounded mesial fold, which becomes very prominent in 

 front : margins of the vah os denticulate. Surface marked by fine radiating 



