May, 1919] Silurian Fossils From Ohio 381 



known as an elongated form of A try pa reticularis. On the 

 ground of its occurrence in Ohio strata, a distinct designation 

 ought certainly to be given to this form, for it is never found 

 above the horizon of the West Union cliff." Although in my 

 opinion the West Union formation does not occur in Greene 

 county, and the rock thus identified there should be referred 

 to the Euphemia dolomite, there is an A try pa of an elongate 

 form which may be regarded as characteristic of the West 

 Union formation. This A try pa is most abundant in the 

 Whitfieldella cylindrica zone, nine feet above the base of the 

 Lower or Bisher member of the West Union formation, in the 

 southeastern part of Hillsboro, Ohio, but specimens occur also 

 in the vicinity of Danville and of Port William, at the same 

 horizon. In general form this variety resembles most closely 

 •the specimen figured by Hall and Clarke (Pal. New York, vol. 8, 

 pt. 2, 1894, pi. 55, figs. 3, 4) from the shaly limestone of the 

 Lower Helderberg group, near Clarksville, New York. 



The amount of elongation of the elongate variety of Atrypa which 

 occurs in the Bisher member varies greatly in different specimens, but 

 is always seen best when the specimen is viewed from the same side as the 

 pedicel valve. In full-grown specimens the beak of the pedicel valve 

 rises rather strongly above the beak of the brachial valve. Both valves 

 are strongly convex. The anterior part of the pedicel valve tends to be 

 slightly flattened and that of the brachial valve to be slightly elevated 

 so that the anterior margin of the shell is slightly nasute. The radiating 

 plications are more narrow than in Atrypa reticularis newsomensis, from 

 the Waldron shale of Tennessee, Kentucky and Indiana. 



Atrypa reticularis hillsboroensis, Var. nov. 

 Plate XVII, Figs. 1 A-D. 



In the Holophragma zone, at the top of the upper or Lilley 

 member of the West Union formation, in the Zink or Corpora- 

 tion quarry, within tjie eastern limits of Hillsboro, Ohio, there 

 is a small form of Atrypa. This variety resembles most closely 

 the variety Atrypa reticularis newsomensis, from the Waldron 

 clay member of the Niagaran. It differs chiefly in its smaller 

 size and in the more crowded condition of the plications. 



Rarely exceeding 17 mm. in length. Anteriorly the pedicel valve 

 tends to be marked by a distinct sinus, while the corresponding part 

 of the brachial valve is elevated into a fold. Frequently the line of 

 junction between the two valves, when viewed directly from in front, is 

 strongly sinuate. Most of the specimens show concentric lines of 



