OF DENISON UNIVERSITY 



77 



Our specimen gapes i)osteriorly and below, but probably the edges 

 naturally apply all around. Greatest convexity upon the post umbo- 

 nal ridge about one-fourth the distance from the beak to posterior 

 angle. A shallow sinus extends from the umbo to the middle of the 

 lower margin. Length 31 mm, height 22 mm, thickness 14 mm, beak 

 about 7 mm, from anterior margin, hinge 23 mm. AUorisma layer, 

 below congl. II, at Newark and Rushville. Not having access to 

 Prof. Hall's description. I am unable to make direct comparisons, but 

 have no doubt that this is the species in question. The comparison 

 with E. tapesiformis. Meek, reveals many differences, that species also 

 seems to come from a different horizon. 



Edmondia huvHngtoiiensis , ^^'- a"«^ ^^'• 

 (Plate IX, Fig. 27, Fig. 17 (.-); Plate IV, Fig. 18.) 



Our species is doubtless the one generally identified with E. bur- 

 lingtonensis and Prof. Winchell has made direct comparison of our 

 forms with Iowa types and sustains the identification. There is, how- 

 ever, a wide range of variation in specimens on the same slab, the 

 majority being almost like E. radiata, H., in form. T\\q original de- 

 scription runs as follows : 



" Shell of medium size, broadly subelliptical in outline, with reg- 

 ularly ventricose valves, breadth ecjual to three-fifths of the length. 

 Beaks situated within the anterior third, strong, prominent, and in- 

 curved. Hinge-line and basal margin gently and equally curved ; an- 

 terior and posterior extremities broadly and equally rounded. Surface 

 marked by numerous strong, concentric undulations, parallel to the 

 margin of the shell. In full-grown individuals there is a shallow, un- 

 defined sulcus, commencing near the centre of the shell, and reaching 

 the border near the middle of the base." " Differs [from E. radiata] 

 in having the posterior extremity narrower than the anterior, instead 

 of much broader; and also in the concentric undulations." 



After an examina*ion of j)erhaps fifty specimens, all from the 

 same bed within a few inches of each other (vertically), we observe 

 the following nearly constant differences. The posterior part of the 

 shell is higher than the anterior ; sulcus well-defined; posterior mar- 

 gin much more broadly rounded than the anterior and oblicjuely sub- 



