524 PALAEONTOLOGY OFHLLLNOIS. 



Surface marked by broad, flattened, scarcely defined plications, of 

 which there are seven or eight on each side of the mesial fold and sinus, 

 with two or three more faintly defined on these parts of the shell, and 

 some appeai'ance of a small plication in the center of the sinus." 



Position and locality : Warsaw, Hamilton, Nauvoo, Niota, and various 

 other places iu Illinois, and Keokuk, Iowa.f_ Keokuk limestone, Lower 

 Carboniferous. 



Genus MYALINA, deKoninck. 



Mtalina Keokuk, Worthen. 



PI 30, Fig. 5. 



Shell of medium size, subquadrate, about once and a half as high as 

 wide, rather oblique ; hinge nearly straight and as long as the greatest 

 breadth of the valves below ; anterior side a little sinuate, posterior 

 side compressed towards the margin, sinuate or deflected inwards imme- 

 diately below the hinge and rounding into the base below ; beak of the 

 left valve pointed, projecting beyond the'hinge and curving forward and 

 inward. Surface marked by distinct, rather irregular lamina?, of which 

 five or six may be counted in the space of half an inch. 



Length of an average sized specimen, 2.2 inches ; breadth, 1.25 inch ; 

 convexity of left valve, about 0.37 inch. 



Position and locality : Keokuk, Iowa, and Warsaw, Nauvoo and 

 Hamilton, Illinois ; Keokuk limestone, Lower Carboniferous. 



Genus PIKNA, Linn. 



Pinna subspatttlata, Worthen. 



PI. 30. Fig. 4. 



This shell is only known from casts of single valves which do not 

 admit of a minute description. It is above the medium size, the speci- 

 mens seen usually ranging from 8 to 10 inches in length, by 1£ in 

 breadth at the posterior end. Valves very gradually tapering, sides 

 apparently flattened towards the posterior end, which seems to have 

 been obliquely rounded, judging from the lines of growth to be seen on 

 the surface of the cast. Cardinal margin slightly thickened with a 

 rather well defined cardinal ridge at the edge. Surface markings 

 unknown. 



In general form it resembles P. spatula of llcCoy, from the Carbon- 

 iferous limestones of Derbyshire, but ours is a larger shell than the 

 European species, and rather wider iu proportion to its length. 



