OBISKANY SANDSTONE. 435 



valve and its four crural processes, which differ essentially from Rhynchtnella, or 

 any other brachiopod known to me. 



I have not discovered the deltidial plates, though they have probably existed. 

 The foramen appears to be formed on the lower side by the umbo of the opposite 

 valve; and, in the young state, there is room for the protrusion of a small pedicle; 

 while in older shells, where these parts have been seen, there appears to be a 

 thickening of the shell, and a closing of the passage to the beak. 



The surfaces of these shells are radiatingly striate or plicate; one of the most 

 conspicuous external features being the broad deep sinus on the lower half of the 

 ventral valve, and the abrupt upward bending of the front of the shell. 



Hitherto I have not distinguished any species in the Lower Silurian rocks; and, 

 in the State of New- York, they are confined to the Lower Helderberg group and 

 the Oriskany sandstone. 



Under this genus, the Eafonia medialis^ E. eminens, E. singularis and E. peculiaris 

 are described on pages 241, 242, 243 and 244 of this volume*. 



• The following illustrations are from the Report of the Regents of the University on the State Cabinet 

 of Natural History for 1858 : 





Fig. 1. The dorsal valve in profile, showing the deep sinus in front, the crural processes (c 1 and c 2); the 

 median septum («), and muscular impression (a). 



Fig. 2. The dorsal valve, looking vertically into the interior. The upper crural processes (c 2) are not quite 

 sufficiently divergent. 



Fig. 3. The ventral valve, showing the form of the muscular impressions (a, RJ and (he teeth (< ). 



Fig. 4. The ventral valve preserving a part of the dorsal valve attached, showing the first crural processes 



(c 1) extending into the muscular cavity. The extension of the shell in front is broken off. to 



show the interior. 



^ 



Fig. 5. Profile of the ventral valve, showing the teeth and the elevation of the shell at R the lower limit of 

 the muscular impression. 



Fig. 6. Another specimen (the figure enlarged) with the dorsal valve downwards, showing the median 

 septum of that valve (»), and the crural processes with the median septum of the ventral valve 

 (») imbraced between the prwesses (c 1). The mu.scular cavity of the ventral valve (R) is 

 limited by inflected lamellae, which embrace the neck of the cardinal process of the dorsal valve. 



