340 PALAEONTOLOGY OF ILLINOIS. 



under the small rudimentary ear. Muscular impressions very 

 faintly marked. Cardinal area rather narrow, and provided 

 with but few longitudinal cartilage furrows. Hinge edentulous. 



This type differs from the typical species of Pterinea, not only in form and 

 o-eneral physiognomy, but in its edentulous hinge, and very obscurely marked 

 or obsolete, anterior muscular impression. It will be readily distinguished 

 fromTrerwTKa, to which the typical species was referred by Prof. Cox, in the 

 Kentucky Geological Report, by being entirely without any traces of the trans- 

 verse cartilage pits characterizing that genus, and indeed of the entire sub- 

 family to which it belongs. 



Whether or not this group should be made to include such forms as Gcrvillia 

 lunulata and G. laminosa, Phillips (Geol. Yorksh., ii, pi. vi.), which have a 

 well developed, distinctly projecting, lobe-like anterior ear, we have no means 

 of determining, until those forms have been more fully characterized. That 

 they are true Gervillias, however, is exceedingly improbable. 



Pterinea (Monopteria) gibbosa, M. and W. 



PL 21, figs. 11, 11a, 116. 



Plerinea {Monopteria) gibbosa, Meek and Worthen, June, 1866. Proceed. Chicago 

 Acad. Sci., p. 20. 



Shell irregularly suborbicular, moderately gibbous ; anterior 

 outline from a little below the middle, forming with the base 

 a more or less regular semicircular curve, and sometimes 

 obliquely truncated, with slightly concave outline in the region 

 of the lunule from the beaks above. Posterior basal margin a 

 little sinuous ; posterior extremity attenuate and angular ; 

 umbonal ridge rather prominent from near the back part of the 

 beak to the produced posterior extremity of each valve ; beaks 

 equal, scarcely projecting above the cardinal margin, and as it 

 were, drawn some distance back of the prominently rounded 

 front. Anterior margins just in front of and under the beaks, 

 abruptly inflected to form the deep lunule ; anterior ear 

 minute, rounded and placed very close up under or between the 

 beaks. Hinge line straight, less than the entire length of the 



