164 Trans. Acad. Sc>. of St. Louis. 



on the valve, they divide at a point about one-third the dis- 

 tance from the beak and each one gives off from its inner side 

 a single branch which occupies the lateral slope of the sinus; 

 cardinal area narrow, with subparallel margins. Brachial 

 valve depressed convex, flattened toward the cardinal extremi- 

 ties ; the fold but slightly elevated above the general surface 

 of the valve, usually marked by a single furrow along its 

 median line, rarely with a lateral one on each side. Surface 

 of each valve marked by twelve to sixteen simple, rounded 

 plications on each lateral slope, and by concentric, lamellose 

 lines of growth which become more crowded near the margin 

 and of which a few are usually stronger than the others. 

 These concentric markings can only be observed in external 

 impressions of the shell. 



The dimensions of an average specimen are: length 11| 

 mm., breadth along hinge-line 25 mm., convexity of pedicle 

 valve 5 mm. 



Remarks. The Burlington specimens of this species were 

 originally included by Hall, along with specimens from the 

 Chonopectus sandstone, in his species 8. biplicatus. The 

 specimens from these two horizons are specifically distinct, 

 however, and the Chonopectus sandstone shell has been re- 

 tained as the typical 8 . biplicatus. Immature individuals of 

 the brachial valves of 8. centronatus, as for example the 

 specimen here illustrated, often have some resemblance to 8. 

 biplicatus, but the pedicle valves of the two species need never 

 be confused. 8. centronatus, although it has an elongate 

 hinge-line with mucronate extremities, never possesses the 

 excessively elongate and attenuate cardinal extremities which 

 are characteristic of S . biplicatus. 



S. centronatus was originally described from the Waverly 

 series of Ohio, but neither the original specimen nor other 

 specimens from the same region, have ever been illustrated. 

 The species has been identified, however, from Nevada, Utah, 

 and the Yellowstone Park, and good illustrations of specimens 

 from these localities have been published. The Burlington 

 specimens agree well with the original description of the spe- 

 cies, and with the published illustrations of western speci- 

 mens. The species is closely allied to 8 . marionensis, but 



