CAUDOmFEROUS PERIOD. 129 



Ventral valve rather shallow, similar to the other valve in number, dis- 

 tribution, and character of the plications ; mesial sinus broad, shallow, and 

 short, generally having two plications at its bottom, but sometimes three ; 

 all the plications in both valves disappeai-ing about the middle, so that the 

 surface of the posterior portion of the shell is almost entirely unmarked 

 except by faint concentric lines of growth. 



Usual length of adult examples about ten millimeters ; breadth, eleven 

 millimeters ; but many much smaller examples are found, and the collec- 

 tions contain a few specimens figured on Plate IX, the dimensions of which 

 are one-third greater than those given, and yet they seem evidently refera- 

 ble to this species. 



This shell has quite a wide geographical range, being found in the 

 Carboniferous strata of Illinois, Iowa, Missouri, Kansas, and Nebraska, and 

 Professor Marcou obtained his type-specimens from near Great Salt Lake, 

 Utah So far as known, it is confined to the strata of the Carboniferous 

 period. It is more nearly related to B. Ottumiva White than to any other 

 shell known to me, the more gibbous examples of which it closely resembles. 



Position and locality. — Strata of the Carboniferous period ; North Fork 

 of Lewiston Caiion, Oquin-h range, and at Meadow Creek, south of Fill- 

 more, Utah. 



Rhynchonella metallica White. 



Plate X, fig. 10 a, h, c, and d. 



Rhynclionella metallica Wliite, 1874, Exp. & Surv. west lOOtli Merit!., Prelim. Eep. 

 Invert. Poss., 20. 



Shell rather less than medium size, depressed, or moderately inflated 

 when advilt ; transversely suboval in outline'; antero-lateral borders abruptly 

 rounded ; front broadly rounded, but usually straightened or slig'htly 

 emarginate at the middle ; postero-lateral margins straightened, laterally 

 compressed, and converging to the beaks at a very obtuse angle. 



Dorsal valve more capacious than the ventral, broadly convex from 

 side to side, a little flattened near the beak, abruptly bent downward at the 

 margins ; mesial fold broad, not much elevated, discernible only on the 

 anterior part of the valve. 



Ventral valve comparatively shallow, slightly convex from side to side ; 

 9 F 



