INVERTEBRATES. 281 



by numerous fine, regular, radiating striae,* about seven or 

 eight of which may be counted in the space of one-tenth of an 

 inch; crossing these, there are, in the visceral region, a few 

 obscure traces of concentric wrinkles. A few scattering bases 

 of spines are also sometimes seen on the ears and anterior 

 slope (in casts) . On some specimens, two or three of the striae 

 on the anterior slope of the ventral valve become more promi- 

 nent and larger than the others. Dorsal valve unknown. 

 Length, from beak to anterior slope, 0.36 inch; do., from the 

 beak over the curve of the valve, to the anterior margin, 0.45 

 inch; breadth at the hinge, about 0.45 inch. 



The extremely fine striae, and produced anterior margin of this shell, will at 

 once distinguish it from any other species of its size and general outline, known 

 to us in our rocks. 



Locality and position : Alton, Illinois ; St. Louis limestone of Lower Car- 

 boniferous series. 



LAMELLIBRANOHIATA. 



Genus MYALINA, deKoninck, 1844. 



(An. Foss. Carb. Belg., p. 125.) 



Mtalina concentrica, M. and W. 



PI. 19, fig. 3a, 3 b, 3 c. 



My alina concentrica, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Sci., Phila- 

 delphia, p. 456. 



Shell small, thin, compressed, subquadrate, somewhat higher 

 than wide, not very oblique; hinge straight, less than the 

 greatest breadth of the valves below; anterior side a little 

 sinuous, ranging at an angle of about 85° from the hinge ; pos- 

 terior side compressed, its margin nearly straight and parallel 

 with the front above, and rounding into the base below ; basal 



* Not represented fine and distinct enough in the engravings. 

 36 Sept. 15, 1866. 



