332 PALAEONTOLOGY OF ILLINOIS. 



gin, 0.80 inch; antero-posterior diameter, 0.73 inch; convexity 

 of the two valves, 0.20 inch. 



This is probably the species described by Dr. Shumard, in the Missouri Re- 

 port, from the Coal Measures of that State, under the name Pccten occidentalis, 

 and subsequently by Prof. Swallow from Kansas, under the name P. Cleavc- 

 landicus. Apparently the same shell was found by Dr. Hayden, in a hard 

 siliceous rock near the Black Hills, Dakota. It likewise occurs in the Upper 

 Coal Measures of Kansas, where it ranges up into beds containing Permian 

 types. 



The engraver was unsuccessful in representing the left valve of this species 

 by figure 5 of our plate. The outline, as there given, is correct, but the costEB 

 are very imperfectly represented. On the shell they are a little flattened, and 

 separated by depressions wider than their own breadth, with usually a smaller 

 one intercalated between each two of the larger, only the latter of which extend 

 up to the beak. Crossing all of these ribs and the depressions between, there 

 are numerous fine, closely arranged concentric striae, And a few stronger con- 

 centric marks of growth, which latter often interrupt or dislocate the costae 

 towards the free borders. These characters are better seen in our figure 5 a. 

 One of the ribs on the posterior umbonal slope of the left valve seems to be 

 always larger than any of the others. 



Locality and position : Upper Coal Measures j Saline Creek, Gallatin county, 

 Illinois. 



Genus STREBLOPTERIA, McCoy. 



Meleagrina (sp.), McCoy, 1844. Carb. Foss. Ireland, p. 80; (not Lam., 1819). 

 Pecten (sp.), McCoy, 1844. lb., and others; (not Muller). 



Streblopteria, McCoy, 1851. Ann. Mag. Nat. Hist., vii, p. 170, (without type or exam- 

 ple); 1855, Brit. Palaeozoic Foss., third part, p. 482. 



"Shell ovate or rounded, obliquely extended towards the 

 anterior side; posterior wing broad, undefined, nearly rectan- 

 gular, extended nearly as far as the posterior margin of the 



Meleagrina. It is traversed by the same fine striae that mark other parts of the area. 

 One of the most important distinctions between this genus, as generally understood, 

 and all of the modern type of the Pcctinidx, is the presence of a distinct, well-defined 

 cartilage pit in the hinge of the latter. The species under consideration, however, 

 shows that there was, sometimes at least, a slight tendency to form a similar cartilage 

 depression in the area of Aviculopeclen, thus furnishing another evidence of the imper- 

 ceptible gradations by which all groups will probably be found linked together when we 

 can have an opportunity to compare very large numbers of the living and extinct types. 



