MAI!Yr>Ax\D (iE()LO(iI(:AL SuRVEY 383 



lamella; which border the Jiiuscular inipret^siuii, and which gradually be- 

 come obsolete in the anterior direction : the muscular impression is oblong- 

 subelliptical, au'l the narrow imprints of the adductor muscles are small 

 and often but faintly defined. There is a niedio-longitudinal septum, 

 which is much less strongly marked than the marginal rim. The deltidial 

 pieces are rarely preserved in the separated valves. 



" The interior of the dorsal valve shows the strong hinge-plates, each 

 with a wide triangular upper surface, and connected by a transverse 

 process a little below the apex, beneath which a foramen passes from the 

 cavity of the valve, opening at the slightly elevated beak in oval aperture. 

 The hinge-plates are extended below along the surface of the shell in 

 strong ridges, which are dichotomized at the beginning of the muscular 

 impression. Anteriorly these plates project in strong crural processes, 

 which are straight only for a short distance, and then diverge in two 

 slender points to the cavity of the ventral valve, in their main direction 

 trending upwards into the dorsal cavity, and then by an abrupt genieula- 

 tion proceed in a converging direction to the commencement of the crural 

 plate. This plate is extremely elongate and very slender, deeply emargi- 

 nate behind, with a slender process proceeding from the center, which, 

 with the two posterior branch.es, converge towards the bottom of the 

 ventral cavity." Hall, 1859. 



R. marylandica is undoubtedly the southern expression of the well- 

 known Upper Oriskany fossil R. oroides (Eaton). In fact, some of the 

 Maryland specimens, if found in New York, would without liesitation be 

 referred to R. oroides. However, the majority of individuals have an ex- 

 pression of their own, and it is advisable to i)re?crve this local variation 

 under the geographic name R. marylandica. This difference in mature 

 specimens consists chiefly in the fact that the latter species is always 

 considerably smaller and more symmetrical than full-grown K. ovoides, 

 but more particularly in the extreme narrowness of the valves and their 

 very broad straight sides. New York R. ovoides also has very wide lateral, 

 nearly plane surfaces, but the degree of development is less and the width 

 of the shell is greater. Even when the great inflection is less pronounced 

 R. inarijlandica is more elongate and less square shouldered posteriorly. 



