148 MISSISSIPPIAN BRACHIOPODA 



valve, and the size and strength of the muscular impressions of both 

 valves. Externally the species differ somewhat in general proportions, 

 contour and surface markings, and although these differences are fre- 

 quently slight,, they correlate with the more marked internal features. 



RHIPIDOMELLA MISSOURIENSIS (Swallow) 

 Plate XX, Figs. 1-8 



1860. Orthis Missouriensis Swallow, Trans.. St. Louis Acad. Sci., vol. 1, 



p. 639. 

 3892. Orthis Missouriensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 



6A, figs. 16-17. 

 1908. Rhipidomella missoiiriensis Rowley, Mo. Bureau Geol. and Mines, 



vol. 8, 2nd ser, p. 78, pi. 17, figs. 43-47. 



Description. Shell lenticular in form, of medium size or a little smaller, 

 suborbicular in outline, usually a little wider than long, the greatest 

 width a little anterior to the middle, the hinge-line much shorter than 

 the greatest width of the shell, the cardinal extremities rounded. The 

 dimensions of two examples of about average size are : length of pedicle 

 valve 19.7 mm. and 18.1 mm., length of brachial valve 19.1 mm. and 18 

 mm., greatest width 23.5 mm. and 20.5 mm., length of hinge-line 8.5 mm. 

 and 9.5 mm., thickness 8 mm. and 9 mm. 



Pedicle valve most convex in the umbonal region, the surface curving 

 somewhat abruptly to the cardinal margin and compressed towards the 

 cardinal extremities, broadly flattened in the median portion of the valve 

 and becoming gently convex towards the antero-lateral and anterior 

 margins ; mesial sinus obsolete or present only as a slight depression 

 close to the anterior margin; beak small, pointed, a little incurved; car- 

 dinal area small, concave, becoming more curved towards the beak, the 

 inferior, flatter portion lying at an angle of about 45 degrees to the plane 

 of the valve; delthyrium wider than high, its width at the base being 

 from one-fourth to one-third the total width of the cardinal area. In- 

 ternally the hinge-teeth are large, strong, and slightly diverging; the 

 flabellate or subovate muscular scars reach anteriorly to the middle, or to 

 a point beyond the middle of the valve, and are limited externally by a 

 slightly raised border, the adductor impressions do not extend to the an- 

 terior margin of the entire scar, but a raised median ridge continues an- 

 teriorly to the limits of the diductor scars ; the unpaired pedicle muscular 

 scar occupies the rostral portion of the valve ; its surface is slightly raised 

 above that of the contiguous scars in front and is sharply differentiated 

 from them; the anterior and lateral margins of the valve are slightly 

 thickened and crenulate internally. 



Brachial valve a little more convex than the pedicle, its greatest con- 

 vexity near or a little posterior to the middle, the surface curving more 



