No. 186.] 



37 



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 pro- 

 trusion 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 closino-'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 bending of the front of the shell. 



Geological range. Hitherto I have not distinguished any species in 

 the Lower Silurian rocks ; and, in the State of New- York, those already 

 known are confined to the Lower Helderberg group and the Oriskany 

 sandstone. 



Under this genus, the Eatonia medialis, E. eviinens, E. singularis and 

 E. peculiaris are described on pages 241, 242, 243 and 244 of the Palae- 

 ontology of New- York, vol. iii ; and E. pumila, E. whitfieldi, and E. 

 sinuata, on pages 437 and 438 of the same volume. 



The following figures are from specimens of Eatonia peculiaris, and 

 illustrate the more important features of the genus. 



Fig. 1. The dorsal valve in profile, showing the deep sinus in front, the crnral processes 

 ( c 1 and c 2 ), the median septum (s), and muscular impression (a). 



rig. 2. The dorsal valve, looking- vertically into the interior. The upper crural pro- 

 cesses ( c 2 ) are not quite sufficiently divergent. 



Fig. 3. The ventral valve, showing the' form of the muscular impressions {a, R) and 

 the teeth (O- 



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 exten-^ 

 sion 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 (s), and the crural processes 

 with the median septum of the ventral valve (s) embraced between tl e 

 processes ( c 1 ). The muscular cavity of the ventral valve {R) is limited 

 by inflected lamellae, which embrace the neck of the cardinal process of the 

 dorsal valve. 



Fig. 7. A cast of the interior of the ventral valve, showing the muscular and vascular 

 itoprfessions. 



