INVERTEBRATES. 349 



ia attaining a much larger size, but in its distinctly sinuous instead of convex 

 basal margin; also in the presence of radiating postero-dorsal costse. 



Locality and position: North branch Saline Creek, Gallatin county, Illinois, 

 in a bed belonging to the Upper Qpal Measures. 



Genus SOLENOMYA, Lamarck, 1819. 



(Solemya, Lamarck, An. Sans. Vert., vol. v.) 

 SOLENOMYA RADIATA, M. and W. 



PI. 26, fig. 10 a, 10 6. 



Solemya radiata, Meek and Worthen, October, 1860. Proceedings Acad. Nat. Sci., 

 Philad., p. 457. 



Shell thin, narrow oblong-oval or elongate subelliptical, 

 moderately convex, nearly closed at each end ; pallial margin 

 rather straight or very slightly contracted along the middle, 

 and rounding up more gradually in front than behind; anterior 

 (longer) side narrowly rounded, its most prominent part being 

 above the middle ; posterior (shorter) side narrowly rounded 

 below and obliquely subtruncate above ; dorsal outline nearly 

 parallel to the base ; beaks much depressed, located less than 

 one-fourth the entire length of the shell in advance of the 

 posterior extremity. Surface with obscure marks of growth, 

 crossed by flat, nearly obsolete, radiating plications, which are 

 sometimes separated, near the middle of the valves, by spaces 

 greater than their own breadth ; plications very oblique and 

 more closely arranged on the anterior side. Length, 1.17 

 inches ; height, 0.47 inch ; convexity, about 0.33 inch. 



Some specimens show the radiating plications nearly as distinctly on the 

 internal cast as on the exterior of the shell. These casts retain distinct impres- 

 sions of an oblique ridge just in advance of the posterior muscular impression, 

 but both the muscular scars seem to be very faintly marked. It is possible our 

 specimens may have been partly closed by pressure in the matrix, though they 

 have the appearance of having been but slightly gaping at either end — more 

 particularly behind. 



