330 PALEONTOLOGY OF OHIO. 



though these shells, as stated, vary much in these characters. Both of 

 the forms figured look unlike Prof. Hall's type of S. opimus, as illustrated 

 in the Iowa Report, being less gibbous and more transverse, with more 

 numerous plications, but in the Illinois Coal Measures there are shells 

 apparently undistinguishable from S. opimus, that seem to shade off into 

 forms like those here figured. In some respects these Ohio shells agree 

 more nearly with S. subventricosus, McChesney, which, however, is gen- 

 erally regarded as a variety of S. ojyimits. 1 have also found it difiicult 

 to distinguish all of these shells from S. Keokuk, var., Hall (Iowa Report, 

 I., part II., 676, plate XXIV., fig. 4a). 



Locality and position: This shell occurs at many localities in the Coal Measures of 

 Ohio, Illinois, Iowa, Missouri, and West Virginia; also numerous places in theEocky 

 Mountain region of tlie far West. 



MOLLUSCA (PROPER). 



LAMELLIBRANCHIATA. 



Genus AVICULOPECTEN, McCoy, 1851. 



(Ann. Mag. Nat. Hist., VII., 171.) 



AvicuLOPECTEN (Streblopteria ?) Hertzeei, Meek. 



Plate 19, figs 13a, b, c. 



Shell usually under medium size, higher than wide, rather compressed, 

 the right valve being nearly flat, and the left only moderately convex; 

 subovate in general outline (exclusive of the small ears), wdth a slight 

 backward obliquity, caused by the greater prominence of the anterior 

 margin ; basal outline semicircular and rounded regularly into the rather 

 prominently rounded anterior margin; posterior margin less prominent 

 than the anterior, and forming a longer and more gentle curve from the 

 posterior ear into the base ; hinge distinctly shorter than the antero- 

 posterior diameter of the valves ; posterior ear in both valves very small, 

 flattened, very obtusely angular, and much shorter than the margin 

 below, from which it is only separated by a faint sinuosity, though it is 

 well defined from the umbo ; anterior ear of each valve distinctly larger 

 than the posterior, though not nearly as prominent as the anterior mar- 

 gin below, rather strongly compressed or flattened, so as to be abruptly 

 separated from the umbo, and in both valves defined by a distinct sinus 

 from the margin below, the sinus being deeper and more angular in 



