SCHELLWIENELLA 67 



placed in Schellwienella because its relationships seem to be closer to 8. 

 planunibona than to any other species, and that form has been definitely 

 established as being one of the more typical members of the genus 



Schellurienella in our faunas. 

 Horizon. Burlington limestone. 



SCHELLWIENELLA BURLINGTONENSIS n. sp. 

 Plate IV, Figs. 1, 2 



Description. Shell large, subsemielliptical in outline, wider than long, 

 the greatest width along the hinge line, the cardinal extremities angular 

 and semiauriculate, the lateral margins gently concave in front of the 

 cardinal extremities, becoming gently convex anteriorly and then curving 

 without interruption into the anterior margin which, with the convex 

 portions of the lateral margins, is nearly semicircular. The dimensions of 

 a pedicle valve are: length 64.5 mm., width along hinge-line 90 mm., 

 height of cardinal area 21.5 mm. The dimensions of a brachial valve are : 

 length 47 mm., width along hinge line 65 mm. 



Pedicle valve gently concave along a line from the beak to near the 

 front margin, beyond which it becomes convex, the surface nearly flat 

 towards the cardinal extremities and convex in a comparatively narrow 

 marginal band along the lateral and anterior margins, most prominent in 

 the umbonal region close to the cardinal margin; mesial sinus obsolete; 

 the beak erect, very broadly obtuse ; cardinal area high and flat, sloping 

 posteriorly from the hinge-line at an angle of about 120 degrees to the 

 plane of the valve, the lateral margins sharply angular, sloping from the 

 beak to the cardinal extremities in a convex curve which is gentle until 

 it approaches the extremities where it becomes much more abrupt; the 

 delthyrium higher than wide, closed by a deltidium. Internal characters 

 not observed. 



Brachial valve convex, the greatest convexity probably near the middle, 

 the surface compressed towards the cardinal extremities, the umbo a 

 little prominent but not protuberant beyond the cardinal margin; mesial 

 fold obsolete; beak scarcely differentiated, not extending beyond the 

 cardinal margin. Internal characters not observed beyond the posterior 

 extremity of the cardinal process which is large, about twice as wide as 

 high, and deeply bifid. 



Surface of both valves marked by regular, rounded, radiating costae 

 about two of which occupy the space of one millimeter near the margin 

 of a mature shell. The costae are crossed by more or less regular con- 

 centric lines of growth of varying strength which become more crowded 

 towards the outer margin. 



