66 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



plane of the edges of the valve; it is divided midway by a narrow 

 pedicle furrow that interrupts the transverse striae of growth. 



Dorsal valve a little shorter and more rounded at the beak ; area 

 short and marked by transverse stride of growth ; both valves mod- 

 erately convex. 



The interior of the ventral valve shows what appears to be a short 

 continuation of the cardinal area forward into the valve before the 

 slope into, the visceral cavity ; it is as though an area with lines of 

 growth was added to the reversed area of the ventral valve of 

 Elkania desiderata (Billings) [1862, p. 69]. The front margin of 

 the area merges in Oholus (Fordiiiia) pcrfectus into the thicker shell 

 back of the visceral cavity, much as in Oholus (F.) helhilus (Wal- 

 cott) [19050, p. 323]. The pedicle furrow extends forward from the 

 posterior margin across the true area and its anterior extension to 

 the visceral cavity. The visceral area is bordered by two ridges 

 that diverge from the sides of the pedicle furrow and extend forward 

 about one-third the length of the valve; these ridges widen toward 

 the front, and where they terminate there appear to be two or three 

 minute muscle scars corresponding to the outside and middle laterals 

 and central scars of Oholus; outside of the ridge there is a furrow 

 that was probably occupied by the main vascular canal, and, beyond, 

 two narrow elongate spaces in which the transmedian and anterior 

 lateral muscle scars appear to be situated ; all the furrows head 

 back against the thickened shell in front of the cardinal area ; the 

 surface of the interior of the valve is marked by concentric lines and 

 very fine radiating striae. 



The dorsal valve has a short, strong median ridge in front of the 

 cardinal area, and well toward the center of the valve a narrow, 

 sharp median ridge ; on each side of the latter, where it begins pos- 

 teriorly, the small, oval, central muscle scar occurs, and, at its an- 

 terior end, the two elongate, oval anterior-lateral scars that are 

 larger than the centrals ; on the thickened postero-lateral portions of 

 the valve the transmedian and outside and middle lateral muscle 

 scars occur close to the outer margin. The surface of the visceral 

 cavity is smooth, but in front of it the minute, irregular vascular 

 markings are very ornate ; a few radiating striae also occur. 



The two interiors described are unusually distinct ; usually the 

 various parts and scars are more or less obscure. 



Observations. — This species approaches Oholus (Fordinia) gil- 

 berti (see p. 65) more nearly than any other species of the genus. 

 It differs in the presence of the sinus in both valves ; in being less 

 convex ; in its less pointed beak, and in its strongly marked interior. 



