Marylaxd Geological Survey 531 



the posterior one ; anterior lobe about the same width as the median one, 

 but less strongly convex. Entire surface of the valves smooth." Waller, 

 190;J. 



The dimensions of an average specimen are : Lengtli 1.75 mm. ; height 

 1 mm. 



Occurrence. — Helderberg Formation, Keyser Mejiber. Tonoloway. 



Collection. — U. S. National Museum. 



Ivlcedenia kummeli (Weller) 

 Plate XCVII, Fig. 16 



Beyrichia kiimmeli Weller, 1903, Geol. Surv. N. J., Pal., vol. ill, p. 266, pi. 



xxiv, fig. 21. 

 Elccdenia kummeli Ulrich and Bassler, 1908, Proc. U. S. Nat. Mus., vol. xxxv, 



p. 301 (gen. ref.). 



Description. — " Carapace subelliptical in outline, a little narrower in 

 front, hinge-line straight, about three-fifths of the total length. Free 

 margin with a depressed, concave border, whose outer edge is slightly 

 elevated in a rounded, marginal ridge, outside of which the surface drops 

 abruptly to the edge of the valve. Valve divided into three lobes by two 

 furrows, which extend ventrally downward from the hinge-line to about 

 the middle of the valve ; the anterior furrow a little shorter than the other, 

 with its lower extremity slightly curved backward. Posterior lobe the 

 largest, including nearly one-half of the valve, with a small, but distinct, 

 tubercle a little in front of its middle point. Middle lobe subglobular in 

 form, more highly convex than either of the others, with a slight, concave 

 excavation at its base. Anterior lobe more depressed convex than either of 

 the others. Surface finely pitted, except on the marginal border and in the 

 furrows; the pits appearing to be angular depressions, with narrow, 

 dividing ridges. 



" The dimensions of the type specimen are : Length 4 mm. ; height 

 2.5 mm." Weller, 1903. 



Occurrence. — Helderberg Formation, Keyser Member. Tonoloway. 



Collection. — U. S. National Museum. 



