INVERTEBRATES. 103 



Beak of the ventral valve pointed, slightly recurved, and ex- 

 tending beyond that of the opposite valve; both valves longer 

 than broad and sinuate in front. Dorsal valve slightly convex 

 at the umbo, where a shallow depression commences and widens 

 downward, and at the front occupies nearly the entire width of 

 the valve. Surface smooth or showing only very fine lines of 

 growth. 



Length of ventral valve from beak to base 7 / 1B inch; greatest 

 breadth 6 / 1H inch; thickness of the shell with the valves united 

 about one-third the length from the beak to the front, % inch. 



Position and locality: From a cherty bed near the base of 

 the Burlington limestone, Pike county, Mo. 



Collector, R. R. Rowley. 



No. 2526 of the Illinois State Museum. 



GENUS ATHYRIS. McCoy. 

 ATHYRIS SQUAMOSA. Worthen. 



PL XI, Fig. 2. 



Athyris sqttamosus, Worthen, March, 1884. Bulletin No. 2 of the Illinois State Museum 

 of Natural History, page 24. 



Shell of medium size, broadly ovate transversely, or nearly 

 circular in outline; valves depressed, and showing externally 

 five heavy imbricating or overlapping plates, that were covered 

 originally with short, coarse spines. Beaks depressed and closely 

 approximate, that of the ventral valve rather the most promi- 

 nent, and projecting beyond the other, with a perforated ex- 

 tremity. 



Length from beak to base 10 / 1H inch^ greatest width of the 

 valves n /i<5 inch; thickness about 5 / 1H inch. This unique form 

 differs so strongly from all others in the Carboniferous forma- 

 tion that a comparison is unnecessary. 



Position and locality: Oolitic beds of the St. Louis limestone, 

 Monroe county, 111. 



Collector A. H. Worthen. 



No. 2527 of the Illinois State Museum. 



