SCHELLWIENELLA 65 



SCHELLWIENELLA PLANUMBONA n. sp. 



Plate III, Figs. 19-22 



1901. Orthothetes sp. Weller, Trans. St. Louis Acad. Sci., vol. 11, p. 181, 

 pi. 16, fig. 1. 



Description. Shell above medium size, subelliptical in outline, usually 

 perfectly symmetrical, broader than long, the greatest width near the 

 mid-length, the hinge-line a little shorter than the greatest width, the 

 cardinal extremities angular. The dimensions of two nearly complete 

 pedicle valves are: length 39.5 mm. and 31 mm., width 49 mm. and 38 

 mm., length of hinge-line 40 mm. and 34.2 mm., height of cardinal area 

 7.8 mm. and 5.8 mm. The dimensions of an imperfect brachial valve are : 

 length 28 mm., width 31 mm., length of hinge-line 28 mm., convexity 8.5 

 mm. 



Pedicle valve concave in general contour, with the small umbonal 

 region depressed convex, and the surface towards the cardinal extrem- 

 ities flattened or gently convex ; mesial sinus obsolete ; the beak obtusely 

 pointed, inclined posteriorly ; cardinal area flat, of moderate height, slop- 

 ing posteriorly from the hinge-line at an angle of about 110 degrees, the 

 lateral margins sharply angular, sloping in nearly straight lines from the 

 beak to the cardinal extremities ; the delthyrium about as wide or a little 

 wider than high, its apical portion covered by a strongly convex deltidium 

 which does not reach much beyond the mid-height of the area, the lower 

 portion apparently being occupied by the posterior extremity of the 

 cardinal process of the opposite valve. The internal features of the 

 valve are not well shown in any of the specimens, but the dental plates 

 seem to be poorly developed, being little more than thickened ridges on 

 each side of the delthyrium ; the muscular scars apparently ill-defined ; the 

 inner surface of the valve crenulate along the lateral and anterior 

 margins. 



Brachial valve convex with the greatest convexity near or anterior to 

 the middle; the surface curving gently from the highest point to the 

 cardinal margin and more abruptly to the anterior and antero-lateral 

 margins, the umbonal region flattened or depressed-convex, the surface 

 compressed towards the cardinal extremities ; mesial portion of the valve 

 not differentiated ; the beak inconspicuous, not extended beyond the car- 

 dinal margin. Internal features of the valve not observed. 



Surface of both valves marked by fine, rounded, radiating costoa which 

 increase by bifurcation and by implantation, from two to four occupying 

 the space of one millimeter. Distinct concentric lines of growth are not 

 commonly present upon either valve. 



Remarks. This species is the most common form of the genus occurring 

 in the Kinderhook oolite at Burlington, Iowa. It is especially character- 



3 



