FOSSILS OF THE COAL MKASriIKS. 573 



cardinal angles not rounded. Dorsal valve moderately 

 convex, mesial fold extending to the beak, and divided 

 along the middle by a shallow, longitudinal depression. 

 Dorsal valve rather deeper than the ventral, the beak ex- 

 tending considerably beyond that of the opposite valve; 

 sinus deep, extending from the beak to the opposite margin, 

 with a single rounded plication in the center. Surface 

 ornamented with sixteen to twenty smoothly rounded pli- 

 cations on each valve, nearly twice as wide as the spaces 

 between them. 



Of tlii* unique little shell we have seen but a single specimen, which 

 was found by Mr. JOHN WOLF, of Canton, in the limestone over one of 

 the coal seams outcropping- at that point. 



Position and locality Lower Coal Measures ; Fulton county, Illinois. 



SPIRIFER CAMERATUS, Morton. 



PI. 25, Fig. 7. 

 Spirifrr eameratus, MORTOX. Am. Jour. Sci.. Vol. XXIX, p. 150; pi. 2, fig. 3. 



Position and locality Common throughout the Coal Measures of Illi- 

 nois. 



LAMELLIBRANCHIATA. 



Genus MOXOTIS. 

 MONOTIS? GREGARIA, M. and W. 



PI. 26, Fig. 5. 

 Monoti* ? gregaria, MEEK and WOKTHEX, 1870. Proceed. Aead Nat. Sci.. Phila.. p. 3?. 



SHELL very small, extremely thin, compressed, oblique, 

 varying from truneato-suboval to subcircular; hinge line 

 less than the breadth of the valves; auricles small, obtusely 

 angular, undefined by any sinuosity of the margins; poste- 

 rior (?) margin rather regularly rounded in outline, and 

 rounding into the pallial margin; anterior (?) border promi- 

 nent below, and rounding into the base, but straight and 



