596 GEOLOGICAL SURVEY OF IOWA. 



small plates in succession, or sometimes one succeeded by 

 two much smaller, which support the inner divisions of the 

 supraradial series. 



First anal plate smaller than the first radials : second 

 range consisting of two hexagonal plates ; third series, three 

 smaller hexagonal plates, succeeded by two plates, and again 

 by a range of two which support the first and second supra- 

 radial plates adjoining the anal area. 



DOME more convex on the anterior side, composed of mi- 

 nute plates having broad grooves running to the interradial 

 spaces. Plates of the calyx joining at their edges by rounded 

 processes, of which there are one, two, or three from each 

 side, leaving oval poriform spaces between, which penetrate 

 to the inner face of the plate, if not quite to the cavity of 

 the body. 



Fig. 6 a. View of anal side of specimen. Fig. 6 b. Anterior side of same-. 



Geological formation and locality. In the Burlington limestone : Burling- 

 ton, Iowa. 



BRACHIOPODA OF THE BURLINGTON LIMESTONE. 

 Orthis michelina, var. burlingtonensis. 



PLATE XII. FIG. 4 a, b. 



SHELL circular or subcircular : valves nearly equally con- 

 vex; hinge-line short. Ventral valve depressed-convex in 

 the middle, flattened below and often gibbous on the umbo, 

 having the beak a little extended beyond the opposite valve ; 

 area high. Dorsal valve regularly convex, sometimes scarcely 

 flattened through the centre : beak a little elevated above 

 the area-margin. 



SURFACE marked by fine bifurcating striae, which have 

 numerous tubular openings : a few strong imbricating lines 

 of growth near the margins. Entire surface punctate or 

 papillose. 



