OF DENISON UNIVERSITY 



83 



ing above the hinge line, anterior umbonal slope steep, a short but 

 well-defined lunule, surface marked only by fine, crowded, concentric 

 stride. Typical measurements as follows : 



Length. 

 (O 



(2) 



24 



(3) 39 



(4) 18. 

 (.0 26 



(6) 24 



(7) 2<) 



(8) 27 



(9) 29 



(10) 17 



(11) 12 



(13) 12 



(14) 10 



Height. 

 1 1 

 10 

 16 



S 



10 



13 

 12 

 12.5 



S 

 6 



Height at y^ from! Thickness of 



posterior margin. 



8 



S 

 12 



6 



8 



8 



9 



9 

 1 1 



6 



4- 5 



4 



both valves. 

 5-5 



Numbers 10-14 ^^e small specimens with slightly different char- 

 acters. The lower margin is straight and the proportional length is 

 less, while the convexity of the surface is more uniform. All the 

 specimens are derived from the shale below the congl. I, and this spe- 

 cies is so abundant and constant that it forms a valuable index to this 

 horizon. It is known from many localities. The form is quite like 

 Nuculana, but Prof. Winchell says: "This has the aspect of Cteno- 

 donta hubbardi, Win., but is too thin and rather too much attenuated 

 behind. However, I have thin s}3ecimens from Holland, near Lake 

 Michigan," 



• 



v4rca ornafa, ^p- "• 



(Plate IX, Fig. 18.) 



Shell small; rather convex, beaks near the anterior third, acute 

 posteriorly, hinge short. Rather convex near the beaks, which are 

 prominent and incurved ; lower line nearly straight, as long as the 

 greatest length of shell; anterior margin slightly curved or nearly 

 straight, intersecting lower line by a sudden curve; posterior outline 

 obliquely truncate forming at the lower angle a regular acute curve ; 

 hinge short, inclined toward the posterior ventral angle : anterior um- 

 bonal ridge high, the sloi)e making a strong angle with the surface of 

 the valves; post-umbonal ridge near the hinge. Surface marked by 



