SP1R1FER 365 



compressed towards the cardinal extremities, with a more or less ill-defined 

 sinuosity passing obliquely from each side of the beak to the lateral 

 margins just in front of the cardinal extremities ; beak small, acuminately 

 pointed, strongly incurved ; cardinal area of medium height, arched and 

 sloping posteriorly, the curvature becoming greater towards the beak, 

 the lateral margins sharply defined, sloping from the beak to the cardinal 

 extremities in a convex curve which becomes more abrupt distally ; lateral 

 slopes each marked by about 25 depressed, rounded plications which 

 bifurcate more or less frequently anywhere between the cardinal and 

 anterior margins, but more frequently towards the beak, these plications 

 grow successively smaller towards the cardinal extremities, the smaller 

 ones sometimes becoming very faint or almost obsolete ; mesial sinus 

 shallow, narrow and sharply defined at the beak, becoming broad and 

 ill-defined anteriorly, marked by a median plication which originates 

 near the beak and which usually divides into two anteriorly, on each 

 side there are four or five plications which originate from the inner 

 margins of the bounding plications or through the bifurcation of plica- 

 tions so originating, making 1.0 to 12 plications altogether in the sinus 

 which are similar in all respects to those of the lateral slopes. 



Brachial valve less convex than the pedicle, the greatest convexity near 

 or posterior to the middle, the surface curving rather abruptly from the 

 umbonal region to the cardinal margin, sloping with a much more gentle 

 curve to the antero-lateral margins, somewhat compressed towards the 

 cardinal extremities; mesial fold defined to the beak, depressed and 

 but slightly raised above the general surface of the valve posteriorly, 

 becoming more elevated and rounded anteriorly but less sharply defined ; 

 marked by plications similar in form and number to those of the opposite 

 valve, all of which originate through the division of a single one at the 

 beak; lateral slopes marked by plications similar in form and number to 

 those of the opposite valve. 



The minute surface markings consist of exceedingly fine longitudinal, 

 rounded corrugations variable in width, the coarser ones generally in 

 the furrows between the plications, from 8 to 12 occupying the space 

 of 1 mm., crossing these are minute sublamellose concentric markings 

 somewhat further apart than the longitudinal corrugations. A few 

 concentric lines of growth are present at irregular intervals. 



Remarks. This species was originally described from the "Waverly of 

 Ohio, and the description given above has been written from such speci- 

 mens. Other specimens from the Chouteau limestone in Missouri have 

 been identified with the Ohio shell, although none have been seen which 

 are so complete or so well preserved as some of the Ohio examples. So 

 far as they are preserved the Chouteau limestone specimens agree closely 

 in almost every respect with the authentic examples of the species, they 



