LOWER HELDERBERG ROCKS. 241 



CrMlJS EaTONIA (n.g.). 



For description and illustration of this genus, see the same under Oriskany 



sandstone. 



Eatonia medialis. 



Plate XXXVII. Fro. I a - y. 

 Mrypa medialis : Vanuxem, Geological Report of the Third District, 1843, pa. 121, f. 4. 



Shell transversely oval, suborbicular or subquadrate : hinge line nearly 

 straight, and forming a very obtuse angle at the beaks. Dorsal valve 

 much larger than the ventral, greatly elevated in the middle (especial- 

 ly near the front), declining with a gentle curve towards the hinge and 

 very abruptly towards the sides. Ventral valve flat or concave, depressed 

 in front so as to form a broad and profound mesial sinus : beak very 

 small, pointed but not prominent, incurved, perforate at the extremity. 

 Surface marked by from twelve to sixteen broad rounded rarely bifurca- 

 ting plications, four of which usually occupy the summit of the mesial 

 fold of the ventral valve, and about three the bottom of the sinus in 

 the dorsal valve : entire surface ( in well-preserved specimens) marked 

 by fine radiating striae, and rarely by a few imbricating lines of growth. 

 The muscular impression in the- ventral valve moderately large, ovate, 

 very distinctly defined by a prominent border, and marked by longitudi- 

 nal slightly radiating plications : near its centre is the small cordiform 

 longitudinally striate impression of the adductor muscle. 

 Associated with this species are a few forms, which, although differing materially 

 from it, I am at present inclined to regard as merely extreme varieties of the same 

 species. Some of these are given on the same plate ( See fig. 1 a - g). In some in- 

 stances (such as 1 c, d, / & g), the plications are almost entirely obsolete, and the 

 valves are compressed together around the front and lateral margins. 



The fine longitudinal striae, as well as the finer concentric strite, are rarely pre- 

 served upon the .specimens which I have seen; though its occurrence upon a few 

 Ijerfect specimens prove this character to have originally existed. In the Eatonia 

 peculiaris and E. singularis, the fine longitudinal striae are characteristic of the 



[ PALiEONTOLOOy III.] 31 



