Maryland Geological Survey 689 



semielliptical or siibtriaii.iiular, a little convex on the ventral side, about 

 twice as wide as liigli ; the lateral margins obtusely angular, sometimes 

 attenuate from compression. Ventral face gently convex, and slightly 

 curving in a longitudinal direction; anterior portion extended in a sub- 

 spatulate expansion. Dorsal face highly convex transversely, and ob- 

 tusely angular along the middle; very slightly concave longitudinally. 

 Aperture oblique, the margin extended on the ventral side ; on the dorsal 

 side the peristome is not fully determined, but is apparently nearly 

 straight on the sides, with a sinus at the angle. Operculum in general 

 form subelliptical, the body of which is moderately convex; the ventral 

 margin (or that corresponding to what may be termed the ventral side 

 of the shell) is regularly ciirving. The xnnbo is situated about three- 

 fourths of the width from the base, and extending thence on each side, 

 almost rectangularly to the tramsverse axis, is a distinct fold which 

 gradually expands towards the margins, and below which is a correspond- 

 ing groove or channel. Above this fold is a nai'row border or flange, 

 which turns upward at a considerable angle (varying in two specimens 

 from ten to more than thirty degrees), and in the center of which is an 

 abrupt angular depression. Shell thin, and usually very imperfectly 

 preserved. Surface of the shell concentrically or transversely striated, 

 the marks often appearing in the casts. In well-preserved specimens 

 these striae are cancellated b}' extremely fine longitudinal ones, which are 

 visible only under a strong lens, and are not preserved in all specimens. 

 These stride are much more conspicuous near the base of the shell, and 

 especially on the ventral margin of the aperture. Surface of the oper- 

 culum distinctly striated concentrically, and also obscurely marked by 

 radiating striae; the margin of the flange is striato-plicate." Hall, 1879. 



Length 14: mm. ; greater diameter 5 mm., lesser diameter 3 mm. 



Occurrence. — Jennings Formation, Parkhead Member. Two and 

 a half miles above mouth of Sideling Hill Creek. 



Collection. — Maryland Geological Survey. 



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