374 Systematic Paleontology 



Eatoxia medialis (Vamixem). 

 Plate LXV, Figs. 31-35 



Atrypa medialis Vanuxem, 1842, Geol. N. Y., Rept. Third Dist., p. 120, text 



fig. 4, p. 122. 

 Eatonia medialis Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 241, pi. xxxvii, 



figs, la-lj/, 1861. 



Description. — Vanuxem did not describe the species, but gave good 

 figures of it and named it the " Medial atrypa (A. medialis) " because he 

 regarded it " as the type or characteristic of the division " Atrypa. 



" Shell transversely oval, suborbicular or subquadrate : hinge-line nearly 

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

 much large than the ventral, greatly elevated in the middle (especially 

 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 bifur- 

 cating 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 strife, 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 longitudinal 

 slightly radiating plications : near its center is the small cordif orm longi- 

 tudinally striate impression of the adductor mutcle.'' Hall, 18.j9. 



The writer has not seen the radiating stri* described by Hall, which 

 when preserved must be very fine. In E. sinuata the radiating strias are 

 well marked and assist to distinguish the species from E. medialis. The 

 latter also has much fewer plications, always three in number, and a well- 

 defined fold and sinus. 



Length about 22 nun. ; width about 27 mm. 



Occurrence. — Hklderberg Formation, New Scotland Member. 

 Dawson, Corriganville, Devil's Backbone, hillside above ]\Iullen"s Quarry 

 Cumberland, 21st Bridge, Maryland; Cedar Cliff, Cherry Eun, West 



