EHIPIDOMELLA 147 



Remarks. This species has been established upon a single unique speci- 

 men, a pedicle valve, which is not unlike, in its general appearance, a 

 somewhat distorted pedicle valve of Productella pyxidata, with which it 

 is associated. The presence of a well-defined cardinal area, however, 

 excludes the specimen at once from the genus Productella. The species 

 is somewhat similar to 8. cymbula, but is smaller, with a relatively more 

 convex pedicle valve, and with the surface spines more erect in position 

 and not arranged in radiating series. 



Horizon. Louisiana limestone. 



Family ORTHIDJE 



Genus RHIPIDOMELLA Oehlert 



Description. Shell subcircular or subovate in outline, biconvex and 

 sublenticular, with the brachial valve somewhat the deeper. Hinge-line 

 short, cardinal areas narrow, especially that of the brachial valve. The 

 pedicle valve with two strong, diverging hinge-teeth, from the base of 

 each a more or less strongly defined curved ridge extends anteriorly, 

 bordering the muscular scar. The muscular area large, flabelliform, and 

 deeply impressed, the margin thickened and elevated; the pedicle scar 

 occupies the entire rostral cavity, the adductor scars rather small, narrow 

 and elongate, entirely surrounded by the large diductor scars, from 

 the narrow adductor scar a median ridge extends anteriorly, dividing the 

 diductors. The brachial valve with deep and narrow dental sockets, the 

 socket plates prominent, sometimes supporting short crura; the cardinal 

 process erect, produced anteriorly in the floor of the valve as a median 

 ridge; the muscular area small, quadripartite, usually indistinct. The 

 surface of both valves marked by fine, rounded, subequal, hollow costae, 

 often opening on the surface, the openings probably representing the 

 broken bases of fine, hollow spines. 



Remarks. In external features the members of this genus are much 

 alike in outline and general contour of the shell, and practically all of 

 them, except R. dubia, might from their external characters alone, be 

 included in the European species R. michelinw as it has been interpreted 

 by European authors. This interpretation of the American form has 

 sometimes been suggested, 1 but a most careful study of many 

 examples has brought out the fact that essentially all of the species 

 which have been described from our American Mississippian faunas are 

 good species, although their most essential characters have not always 

 been pointed out, and that several additional species must be recognized. 



The most important characters for use in the differentiation of species 

 are the form of the cardinal process and socket plates of the brachial 



iWeller, Bull. Geol. Soc. Am., vol. 20, p. 296 (1909). 



