114 PALAEONTOLOGY OF ILLINOIS. 



AVICULOPECTEN MONROENSIS. Worthen. 



PI. XXII, Fig. 8-8a. 



Aviculopecten monroensis Worthen, March, 1884, Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 21. 



Shell below the medium size, left valve moderately convex, 

 sub-ovate or nearly circular below the ears, the lateral borders 

 gradually rounding into the broadly rounded basal margin. 

 Ears nearly equal in size and ornamented with five or six rad- 

 iating costse similar to, and continuous with those on the body 

 of the shell, and also with delicate concentric lines that are 

 plainly visible under an ordinary lens. Surface of the valve or- 

 namented with 60 to 70 irregular sized rounded costae that are 

 usually a little wider than the intervening spaces, and fine 

 transverse striaa similar to those on the ears, which are only 

 visible under a lens. A right valve belonging apparently to 

 this species is nearly flat, radiating costa? very fine, anterior 

 ear longer than the other and defined by a deep narrow sulcus, 

 and both ornamented with costse and concentric striae that are 

 a litttle less conspicuous than those on the opposite valve. 



Length of left valve 1:1 / 16 inch; breadth about the same; 

 length of hinge line / 16 inch. 



Position and locality: Oolitic beds of the St. Louis group, 

 Monroe county, 111. 



Collector, A. H. Worthen. 



No. 2509 of the Illinois State Museum. 



AVICULOPECTEN TALBOTI. Worthen. 



PI. XXII, Fig, 11-lla. 



Aviculopecten talboti Worthen, March, 1884. Bulletin No. 2 of the Illinois State 

 Museum of Natural History, page 21. 



Associated with the A. monroensis, there is a smaller form 

 that seems to be clearly distinct, and may be characterized as 

 follows : 



Shell small, broadly ovate in outline, left valve moderately 

 convex, ears short and not well defined from the lateral bor- 

 ders. Surface apparently smooth, but under a good lens min- 

 ute radiating and concentric lines may be seen, which are most 

 conspicuous on the ears and lateral borders of the shell. 



