OMSK ANY SANDSTONE. 
445 
extending from beneath the thickened cardinal process for one-third the 
length of the shell. The crural processes are comparatively long and 
slender, and, at their base, are separated by a prominent callosity. In 
older shells this callosity becomes thickened and duplicated; and the 
median groove in the cardinal process at the base of the crura, lying 
below the plane of the beak, becomes filled up, and the process extended 
above the beak, with an oval impression on each side. 
This beautiful shell is remarkable for its regular ovoid form and vertically 
compressed sides. It differs from any other species of equal size known to me, by 
the entire absence of a sinus in either valve. Adult specimens appear to have been 
generally higher than wide, though in younger individuals the width is greater 
than the height. 
Fig. 1 a. Dorsal view of a specimen of medium size. 
Fig. 1 b. Profile of the same, showing some parasitic bodies attached at the front and sides. 
Fig. 2 a. Dorsal view of a larger individual. 
Fig. 2 b & c. Profile and front views of the same. 
Fig. 2 d. Cardinal view, showing the deep sinuosities in the dorsal valve and the auricular 
extensions of the ventral valve. 
Fig. 3. A partial cast of a large individual of this species. 
Fig. 4 a. The ventral valve, showing the muscular impression : the teeth are broken off. 
Fig. 4 b. A fragment of a dorsal valve, showing the muscular impression, teeth, etc. 
Fig. 4 c. Cardinal view of the preceding specimen. 
Fig. 5. The rostral portion of a specimen, showing the junction of the two valves, the median 
septum in the dorsal valve, and the crural processes of the ventral valve. 
Fig. 6 a. The interior of the dorsal valve, showing the median septum, the crural processes, 
the thickened cardinal process with the prominent bilobed summit, and the dental 
fossets. 
Fig. 6 b. Cardinal view of the preceding specimen, showing the sinuosities on each side of 
the beak, the cardinal and crural processes. 
Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 
