12 



above the hinge Hue. Umbones slightly depressed, with an uu- 

 defined sulcus uear the beaks, which fades out ou the eouvex part 

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

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

 basal margin. Surface marked by concentric, lamellose lines of 

 growth and finer radiating stri.-u that are plainly visible ou 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 K. A. Blair and S. A. Miller, in the Chouteau liuie- 

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

 ler. The specific name is in honor of tlie veteran collector, li. 

 A. Blair. 



Family AVICULID.E. 



POSIDONOMYA LASALLENSIS, n. sp. 



Plate I, Fig. 17, left valve; Fig. IS, same magnified tiro diameters. 



Species medium size. Shell subovate. Anterior margin ob- 

 li(jucly 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 oblitjue undefined sulcus. Beak anterior to the middle of 

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

 margin. Umbones convex, and mtrging 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 dirt'erent from all other forms that have been 

 referred to Posidonomija 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 tiie family Ariciilida-. 



Found in the t^oal Measures at La Salle, Illinois, anil now in. 

 the collection of Wm. F. F. Gurley. 



