12 



above the hinge line. Umbones slightly depressed, with an un- 

 defined sulcus near the beaks, which fades out ou the convex part 

 of the shell, and does not produce a constriction at the basal 

 margin. Post-umboDal slope subangular and extends to the post- 

 basal margin. Surface marked by concentric, lamellose lines of 

 growth and finer radiating stride that are plainly visible on the 

 casts, to the unaided eye, but on the shell itself presents a beau- 

 tiful cancellated ornamentation. 



This is a very handsome and marked species that cannot be 

 mistaken for any other. 



Found by E. A. Blair and S. A. Miller, in the Chouteau lime- 

 stone, near Sedalia, Mo., and now in the collection of S. A. Mil- 

 ler. The specific name is in honor of the veteran collector, R. 

 A. Blair. 



Family AVICULIDiE. 



rOSIDONOMYA LASALLENSIS, U. sp. 



Plate I, Fig. 17, left valve; Fig. 18, same magnified two diameters. 



Species medium size. Shell subovate. Anterior margin ob- 

 liquely truncated in front of the beaks, and then vertically, so as 

 to leave a small ear in front, and then broadly rounded, which is 

 continued regularly into the basal margin. Hinge line straight 

 posterior to the beaks, slightly elevated and terminates in an ob- 

 tuse extremity. Posterior margin below the wing broadly rounded 

 which is continued regularly into the evenly rounded base. The 

 posterior wing is flat, and separated from the body of the shell 

 by an oblique undefined sulcus. Beak anterior to the middle of 

 the shell, acute but not projecting much if any above the hinge 

 margin. Umbones convex, and merging into the general con- 

 vexity of the shell. Surface marked by six or seven distant, ele- 

 vated, concentric rounded ridges that do not appear as concentric 

 undulations of growth, but as distinct lines of surface ornamen- 

 tation. Between these rounded ridges there are numerous fine 

 concentric lines. 



This species is so different from all other forms that have been 

 referred to Posidonomya that it is with some hesitation we refer 

 it to that genus. It is an aviculoid shell and seems to be nearer 

 to that genus than to any other in the family Aviculidce,. 



Found in the Coal Measures at La Salle, Illinois, and now in 

 the collection of Wm. F. E. Gurley. 



