SCHUCHERTELLA 55 



other members of the genus, is to be found in the comparatively narrow 

 costas separated by broad intercostal spaces. 

 Horizon. Fern Glen formation. 



SCHUCHERTELLA LENS (White) 

 Plate III, Figs. 1-8, 9? 



1862. Streptorhynchus lens White, Proc. Boston Soc. Nat. Hist., vol. 9, 



p. 28. 

 1892. Orthothetes lens Hall and Clarke, Int. to Study of iBrach., pt. 1, 



pi. 16, figs. 12-16. 

 1892, Orthothetes lens Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 11A, 



figs. 16-22. 

 1894. Streptorhynchus lens Keyes, Mo. Geol. Surv., vol. 5, p. 67, pi. 39, 



figs 2a-b. 



1904. Schuchertella lens Girty, Proc. U. S. Nat. Mus., vol. 27, p. 734. 

 1908. Orthothetes lens Rowley, Mo. Bureau Geol. and Mines, vol. 8, 2nd 



ser., p. 78, pi. 17, figs. 37-42. 



Description. Shell small, lenticular in form, broader than long, the 

 greatest width near the mid-length of the shell, the hinge-line usually a 

 little shorter than the greatest width, the cardinal extremities angular. 

 The dimensions of two individuals are : length of pedicle valve 14.3 mm. 

 and 14.7 mm., length of brachial valve 12.9 mm. and 13.1 mm., greatest 

 width 16.8 mm. and 17 mm., length of hinge-line 14.7 mm. and 13 mm., 

 thickness 6 mm. and 5.5 mm., height of cardinal area 3.8 mm. and 2.5 mm. 



Pedicle valve depressed convex, most prominent in the umbonal region, 

 the surface sloping with a gently convex curvature to the lateral and 

 anterior margins, not at all or only very slightly depressed towards the 

 cardinal extremities; mesial sinus obsolete; beak obtusely pointed, not 

 incurved ; cardinal area nearly flat, with sharply defined lateral margins, 

 sloping posteriorly from the hinge-line at an angle between 115 and 125 

 degrees to the plane of the valve ; delthyrium broadly triangular, as broad 

 or broader than high, closed by a rather strongly convex deltidium 

 which is deeply concave along its cardinal margin. Internally the apical 

 portion of the valve is more or less solidified, the cardinal teeth are un- 

 supported by dental plates, although the inner surface of the valve is 

 thickened along each side of the delthyrium; the muscular scars are of 

 moderate size, subovate in outline with their anterior margin lying 

 posterior to the middle of the valve, the central adductor scars are 

 variable in their differentiation, sometimes being only faintly defined and 

 again being sharply defined and sometimes raised anteriorly above the 

 level of the diductor scars on either side ; the inner margin of the valve 

 is crenulated laterally and anteriorly. 



Brachial valve less convex than the pedicle, the greatest convexity 

 posterior to the middle, the surface compressed towards the cardinal ex- 



