156 MISSISSIPPIAN BRACHIOPODA 



of hinge-line 5 mm., thickness 4.3 mm>. The length of one of the largest 

 pedicle valves observed is 12 mm. 



Pedicle valve depressed convex, the median portion of the valve broadly 

 flattened from the beak anteriorly, the surface curving rather abruptly 

 to the cardinal margin and much more gently to the lateral and anterior 

 margins, very slightly or not at all compressed towards the cardinal 

 extremities; mesial sinus originating near the mid-length of the valve 

 and becoming rapidly deeper towards the front where it forms a rather 

 conspicuous depression in the larger individuals; it is rounded in the 

 bottom and ill-defined laterally; the beak is small, pointed and only 

 a little incurved ; the cardinal area is small, moderately incurved, of mod- 

 erate height, and is not very sharply defined at its lateral margins; the 

 delthyrium is large and open and occupies fully one-half the entire car- 

 dinal area. Internally the hinge-teeth are rather large and strong, and 

 are supported by very short dental lamellae ; the flabellate muscular scars 

 are large, reaching anteriorly beyond the middle of the valve, sometimes 

 extending four-fifths the total length of the valve, the adductor scars are 

 small, usually depressed below the general surface posteriorly, but ele- 

 vated anteriorly and extended to the anterior margin of the diductor 

 scars as an elevated ridge with gradually converging sides, the diductor 

 scars very large and generally marked by radiating ridges. 



Brachial valve a little deeper than the pedicle and more regularly con- 

 vex, the surface curving rather abruptly to the cardinal margin and more 

 gently to the lateral and anterior margins, the mesial portion of the valve 

 sometimes slightly flattened, but usually not differentiated from the gen- 

 eral curvature; the beak small, obtusely pointed and not incurved; the 

 cardinal area very small and lying in nearly the plane of the valve. In- 

 ternally the cardinal process is large, its posterior surface is divided by a 

 median ridge which slopes from the highest point towards the beak, the 

 anterior surface drops nearly vertically to the floor of the valve, along 

 which the process is continued as a median ridge of greater or less strength 

 to about the middle of the valve ; the dental sockets are large and deep, 

 the socket-plates are prominent and are produced anteriorly into slightly 

 divergent, crura-like processes; the muscular scars are quadripartite, 

 being smaller and much less deeply impressed than those in the pedicle 

 valve. 



The surface of both valves marked by fine, rounded, radiating costse 

 which increase by bifurcation, from four to five occupying the space of 

 one millimeter. More or less inconspicuous, concentric lines of growth 

 are also usually present. 



Remarks. This is one of the smaller species of the genus, being com- 

 parable in size to R dubia*. and R. jerseyensis only. It differs from R. 

 jersey ensis, with which it is similar in general form, in the conspicuous 

 mesial flattening of the pedicle valve, and the well developed mesial sinus 



