EHIPIDOMELLA 149 



abruptly to the cardinal margin, slightly compressed towards the car- 

 dinal extremities, towards the anterior margin the surface is gently 

 convex and towards the lateral margins a little more strongly so; the 

 median portion of the valve is slightly flattened and is not infrequently 

 a little impressed along the median line to form a very shallow, ill- 

 defined sinus, which, with the slight flattening of the opposite valve, 

 sometimes makes the outline of the shell a little emarginate in front ; the 

 beak is small, obtusely pointed and scarcely incurved; the cardinal area 

 is a little smaller than that of the opposite valve and lies in nearly the 

 plane of the valve. Internally the cardinal process is rather large and 

 is continued as a flattened median ridge along the floor of the valve 

 dividing the quadripartite muscular impression ; the socket plates are 

 prominent and widely diverging, with deeply excavated dental sockets; 

 the muscular scars smaller and less deeply impressed than those of the 

 opposite valve. 



The surface of both valves marked by nearly uniform, rounded costag, 

 about three or sometimes four of which occupy the space of one milli- 

 meter, and which increase by bifurcation and intercalation ; the shell sub- 

 stance is pierced by radiating canals or tubes which follow the direction 

 of the costae and which open at intervals to the exterior along the tops of 

 the costse. Regular, concentric lines of growth are usually present which 

 often become more or less crowded towards the front margin. 



Remarks. This species may be distinguished from R. burlingtonensis, 

 which it resembles, by its proportionally broader form, the smaller and 

 less extended beak of the pedicle valve, by the somewhat more strongly 

 developed radiating costae and by the less frequent openings of the radial 

 canals upon the shell surface, all of which are of uniform size. The size 

 of the species is usually less than that of R. burlingtonensis. 



Horizon. Louisiana limestone. 



RHIPIDOMELLA BURLJNGTONENSIS (Hall) 

 Plate XXI, Figs. 5-7; Plate LXXXIII, Figs. 7-8 



1858. Orthis michelini var. burlingtonensis Hall, Geol. Iowa, vol. 1, pt. 2, 



p. 596, pi. 12, figs. 4a-b. 

 1874. Orthis Michelini var. Burlingtonensis? "White, Prelim. Rep. Inv. 



Foss., p. 17. 

 1892. Orthis Burlingtonensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 



pi. 6A, fig. 13. 

 1892. Orthis (Rhipidomella} Burlingtonensis Hall and .Clarke, Pal. 



N. Y., vol. 8, pt. 1, pi. 20, figs. 5-6. 

 1894. Orthis burlingtonensis Keyes, Mo. Geol. Surv., vol. 5, p. 63, pi. 38, 



fig. 7. 



Description. Shell of medium size or larger, subovate in outline, the 

 length and width nearly equal or wider than long, the greatest width 



