BRACHIOPODS. 97 



examples somewhat less, than the breadth; cardinal margins 

 sloping at various angles from the beaks ; lateral margins more 

 or less rounded, or obtusely subangular; front rather irregularly 

 rounded. Dorsal valve nearly flat on each side, and more or 

 less concave in the middle ; beak not incurved. Ventral valve 

 convex along the middle, and sloping to the sides ; beak in- 

 curved a little beyond the hinge. Surface ornamented by about 

 seven to ten rounded plications on each valve, two of which, 

 on the middle of the ventral valve, are larger and more promi- 

 nent than the others, while the middle one on the other dies 

 out before reaching the beak. Crossing the whole are distinct, 

 regularly arranged, imbricating lamellae of growth. 



Horizon and localities Upper Silurian limestone : Bai- 

 ley landing ( Perry county ). 



Atrypa occidentalis HALL. 

 Atrypa aspera, of American authors. 



Atrypa aspera^ var. occidentalis Hall, 1858: Geology Iowa, vol. I, p. 515, 

 pi. vi, figs. 3a-d. 



Shell of medium size, longitudinally subovoid, inequivalve. 

 Ventral valve flattened, somewhat convex in the umbonal re- 

 gion; beak small, closely incurved; foramen very minute. 

 Dorsal valve very convex, often hemispherical ; beak closely 

 incurved. Surface marked by large, rounded folds, crossed at 

 rather regular intervals by elevated, curved lamellae, which are 

 often produced into short, tubular spines. 



Horizon and localities. Devonian limestone : Win field 

 (Lincoln county ). 



Atrypa reticularis (LINN.EUS). 



Plate xli, figs. l?a-b 



Anomia reticularis Linnaeus, 1767 : Syst. Nat., vol. I, p. 1152. 

 Atrypa reticularis Dal man, 1827 : Vet. Akad. Handl., p. iv, fig. 2. 

 Atrypa reticularis Hall, 1858 : Geology Iowa. vol. I, p. 515, pj. vi, figs. 4a-e 

 and 5a-c. 



Differs from A. occidentalis chiefly in the larger size, much 

 finer radiating costae, and less inbricated character of the con- 

 centric lines of growth. 



Horizon and localities Devonian, Callaway limestone : 

 Fulton ( Callaway county ). 

 G-8 



