66 Trans. Acad. Sci. of St. Louis. 



men is an imperfect, badly crushed example, which when 

 complete must have had a diameter of about 25 mm. Any 

 really distinguishing characters by which it may be separated 

 from various other species of the same genus in the Devonian 

 and Carboniferous faunas, are not well preserved. The 

 description of the species given above is a copy of the 

 original one written by White. 



Orthothetes inaequalis (Hall). 



PI. I. f. 18. 



Shell subplano-convex or slightly concavo-convex, sub-ellip- 

 tical or subsemicircular in outline, more or less unsymmet- 

 rical; the hinge-line usually equaling the greatest width of 

 the shell, but sometimes a little less. Pedicle valve nearly 

 plane, slightly concave or convex; usually convex near the 

 beak and concave in all directions to the lateral and anterior 

 margins. Brachial valve strongly convex, greatest convexity 

 near the middle. Surface of both valves marked by fine, 

 radiating costae which are somewhat irregular, and often 

 alternate in size. More or less conspicuous concentric lines 

 of growth are often present on one or both valves, though 

 they are usually stronger on the pedicle valve. The dimen- 

 sions of an average specimen are: length, 19 mm., and 

 breadth, 26 mm. 



Remarks. A study of a large number of specimens from 

 many localities and horizons, will probably show this species 

 and perhaps others of the same genus in the lower Carbon- 

 iferous faunas, to be not distinct from O. chemungensis which. 

 is so common throughout many of the Devonian faunas. 

 Because of their great amount of variation, it is extremely 

 difficult to draw sharp specific lines between the members of 

 this genus, and this can only be done when a large number 

 of specimens from many localities shall be brought together 

 and diligently studied. 



SCHIZOPHORIA SWALLOVI (Hall). 



PL I. f. 11-13. 



Shell subcircular or transversely subelliptical in outline; 

 hinge-line shorter than the greatest width of the shell, the 



