90 



[ Senate 



EaTONIA MEDIALIS. 



Atrypa medialis, Vanuxem. 



Pal.N.Y. Vol. iii, pi. 37, f. 1. 



Shell transversely oval, suborbicular or subqiiadrate ; hinge 

 nearly straight, and forming a very obtuse angle at the beaks : 

 dorsal valve much larger than the ventral, greatly elevated in 

 the middle (especially near the front), declining with a gentle 

 curve laterally and towards the hinge : 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 bifurcating plications, 

 four of Avhich usually occupy the summit of the mesial fold of 

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

 the dorsal valve : lines of growth obscure. The visceral im- 

 pression in the ventral valve moderately large, ovate, very 

 distinctly defined by a prominent border, and marked by lon- 

 gitudinal 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, b, c, d, e, f, g). In some instances ( such as 1 c, d, f & g ), 



EatoXIA MEDIALia. 



