346 Report of the State Geologist. 



The traces of the surface striae preserved show them to have 

 been very fine and numerous. 



Keokuk limestone. New Providence, Indiana. 



This species is similar in some general respects to Derhya 

 Broadheadij but differs in its narrower and lower cardinal area, 

 less convex umbo on the brachial valve and in the absence of a 

 median sinus on this valve. It may be compared with the Strep- 

 torhynchus crenistria, var. senilis, Phillips (Davidson), from the 

 lower Carboniferous of Great Britain. 



Derbya? costatula. 



Plate 5, fig. 9. 



Derhya ? costatula, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, p. 346, pi. 

 xi b, figs. 16, 17. 1893. 



Shell small, outline semioval. Hioge-line nearJy equal to the 

 greatest diameter of the valve. Cardinal area moderately high, 

 with a prominent deltidium very wide at the base. Pedicle- 

 valve with an elevated beak from which the surface slopes to the 

 margins with a tendency to irregular growth. Brachial valve 

 faintly depressed at the umbo, but otherwise pretty regularly 

 c ^nvex, the most elevated point being a little behind the middle 

 of the valve. There is a faint median sinus over the anterior 



Surface marked with a few coarse radial ribs, between each 

 two of which are implanted one, two or three much finer ones. 

 These ribs are crossed by a few distinct concentric varices of 

 growth. 



Chester limestone. Crittenden county, Kentuchy. 



This species is readily distinguished by the character of its sur-- 



face ornamentation, and though the interior features of the shell 



are as yet unknown, a very closely allied form from the upper 



Coal Measures near Kansas City has a well developed median 



septum in the pedicle- valve, and is hence to be referred to the 



ffenus Derbya. 



Derbya Broadheadi. 



Plate 5, figs. 15, 16. 



Derb7ja Broadheadi, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, p. 347, 

 pi xi a, figs. 23, 24. 1892. 



Shell with irregularly suboval marginal outline. Hinge-line 

 short, its length not exceeding and usually less than one-half the 



to 



