82 MISSISSIPPIAN BRACHIOPODA 



between them a median ridge develops and continues anteriorly some- 

 times reaching the middle of the valve ; towards the anterior border and 

 extending to the lateral borders, the inner surface of the valve is papil- 

 lose, the papillas being arranged more or less regularly in radiating lines, 

 and the inner border of the valve is costate, the costae corresponding with 

 those of the external surface. 



The surface of both valves is marked throughout by fine, nearly regu- 

 lar, radiating costae, which increase by bifurcation on the pedicle valve 

 and by intercalation upon the brachial valve. The total number of costae 

 varies from 175 to 225, according to the size of the individual, but the size 

 of the costae is nearly constant, about six occupying the space of one 

 millimeter, except near the beak, where they are somewhat finer. 



Remarks. The typical horizon for this species is the Burlington lime- 

 stone, where it sometimes occurs in great numbers. Ordinarily the aver- 

 age size of the species is about 15 mm. or a little more in width. 



In Worthen's original description of the species the number of costas 

 are said to be 100 to 120 or more, but this number is understated, as has 

 been determined from the examination of authentic examples from the 

 typical locality. In Meek and Worthen's description of the species 12 

 to 14 costae are said to be present in .10 inch, which would be nearly the 

 number observed on specimens studied by the writer, 6 costae in 1 mm. 

 would be equivalent to 15 in .10 inch. Winchell's C. multicosta is a 

 closely allied species, but grows to a larger size ; it also has a longer, hinge- 

 line and more angular, cardinal extremities than C. ttlinoisensis, and 

 usually has somewhat finer costa?. C. shumardiana DeKoninck, with 

 which C. illinoisensis has sometimes been confused, is an entirely different 

 species, and a comparison of the two forms is made under the description 

 of that species. 



Horizon. Burlington limestone, Keokuk limestone. 



CHONETES MISSOURIENSIS n. sp. 

 Plate VIII, Fig. 20 



Description. Shell transversely subelliptical in outline, the hinge-line 

 a little shorter than the greatest width of the shell, the cardinal extremi- 

 ties obtusely angular or a little rounded. The lateral margins gently con- 

 vex posteriorly, curving more rapidly towards the front and passing 

 without interruption into the anterior margin which is gently convex in 

 the middle. The dimensions of the holotype are : length 16.7 mm., width 

 26 mm., convexity 5 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the beak and gently to the cardinal margin, becoming com- 

 pressed towards the cardinal extremities ; the median portion of the valve 

 flattened or slightly depressed in a broad, shallow, ill-defined median 



