188 aASTEROPODS. 



present requirements, at least insofar as the American forms 

 are concerned. 



The following American shells are regarded as the most 

 typical representatives of Orthonychia : 



Platyceras subrectnm, Hall. P. incile, Hall. 



P. dentalium, Hall. P. tubaeforme, Hall. 



P. concavum, Hall. P. curvirostrum, Hall. 



P. attenuatum, Hall. P. agreste, Hall. 



P. unguiforme, Hall. P. arcuatum, Hall. 



P. lamellosum, Hall. P. tortuosum. Hall. 



P. spirale, Hall. P. cyrtolites, McChesney. 



P. acutirostrum, Hall. P. chesterense, Meek & Worthen. 



P. formosum, Keyes. 



Orthonychia spirale (HALL). 



Platyceras spirale Hall, 1859 : Pal. New York, vol. Ill, p. 331, pi. Ixiii, 



tigs. 4-9. 

 Platyceras spirale Meek & Worthen, 1868 : Geol. Sur. Illinois, vol. Ill, p. 



389, pi. vii, figs. 2a-c. 



Shell small, elongate, completely uncoiled, gradually in- 

 creasing in size from the apex ; body- whorl with a few broad 

 longitudinal folds. Aperture subcircular, margin sinuous. 

 Surfac% smooth, marked only by lines of growth. 



Horizon and localities. Upper Silurian limestone : Bai- 

 ley landing ( Perry county ). 



Orthonychia cyrtolites ( MCCHESNBY). 



Plate liii, flg 15. 



Platyceras cyrtolites McUhesney, 1860: Desc. New Pal. Foss. West. 



States, p. 71. 



Platyceras cyrtolites Keyes, 1889 : Proc. Acad. Nat. Sci., Phila., p. 288. 

 Capulus cyrtolites Keyes, 1890: American Geologist, vol. VI, p. 9. 

 Capulus cyrtolites Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 167, pi. 



ii, fig. 2. 



Shell small, slender, arched ; composed of about one volu- 

 tion ; dorsally sub-angular, with a broad flattened area on each 

 side; posteriorly somewhat plicate. Apical portion small, 

 incurved, sometimes enrolled or contiguous. Aperture mod- 

 erately large, sub-quadrangular; lip sharp, sinuous. Surface 

 marked by strongly undulating lines of growth, which are often 

 somewhat imbricated. 



