Weller — Kinderhook Faunal Studies. 177 



the termination of the sharp postumbonal ridge, from which 

 the surface descends precipitously to the truncate posterior 

 margin. Hinge-line short, rounded, edentulous. Greatest 

 thickness a little above the middle of the shell. Surface 

 marked only by faint incremental striae." 



Length of the type specimen 21 mm., height 1\ mm., and 

 convexity of right valve 6 mm. 



Remarks. In the University of Michigan collection, two 

 distinct species of shells have been associated as the types 

 of S. trigonalis. The authentic type, judging from Winchell's 

 description, and the measurements given by him, is the 

 larger specimen of the two accompanying illustrations. The 

 other specimens attached to the same card with this one in 

 the Michigan collection, are much smaller and are from the 

 Chonopectus sandstone rather than from the bed under dis- 

 cussion. In a former contribution these Chonopectus sand- 

 stone specimens have been described as a new species, 

 Schizodus iowensis* 



Promacrus cuneatus Hall. 



Trans. Acad. Sci. St. Louis. 10: 104. pi. IV. f. 20. 



Original description. " Shell below the medium size, elon- 

 gate, attenuate, subcuneate anterior to the beak. 



" The specimen is a fragment, preserving the anterior end 

 and the beak. It proves, upon comparison with Promacrus 

 Missouriensis, to belong to the same genus. It is distin- 

 guished by its smaller size, stronger and more regular con- 

 centric undulations, and distinct continuous radii of the sur- 

 face, which become nodose at their intersections with the 

 concentric undulations. 



"The specimen, anterior to the beak, has a length of 45 

 mm. and a height at the beak of 24 mm." 



Remarks. The horizon of this species, as cited by Hall, is 

 simply "Yellow Sandstones at Burlington, Iowa." At the 

 time of publication of the description of the Chonopectus 

 sandstone fauna, no specimen of this species had been ob- 

 served and it was provisionally included in that fauna and a 

 copy of Hall's original illustration was reproduced. Since 



* Tran*. Acad. Sci. St. Louis. 10: 101. 



