PLATE V. 



OrTHOTHETES DE8IDEEATUS, Hall. 



Page 345. 



Fig . 1 . A cardinal view of an internal cast . 



Fig. 2. A dorsal view of the same specimen, showing the dorsal muscular 

 scars, 

 Waverly group . Ohio. 



Derbya Bennetti, Hall. 

 Page 347. 



Fig. 3. Transverse section near the apex of the pedicle-valve, showing the 

 median septum coalesced with a solid callosity filling the apical portion 

 of the deltidial cavity . 



Fig. 4. A section of the same specimen nearer the hinge. This shows the 

 dental ridges, tips of the cardinal process and the median septum. 



Figs. 5, 6. Cardinal and profile views of the same specimen. 



Figs. 7, 8. Dorsal and ventral views of a smaller shell, with a large scar of 

 attachment. 

 Upper Coal Measures. Kansas City, Missouri. 



Derbya (?) costatula, Hall. 



Page 346. 



Fig. 9. Dorsal view, showing the characteristic surface ornamentation of the 

 species, x 2. 

 Chester limestone. Crittenden county, Kentucky. 



Debbya affinis, Hall. 



Page 349. 



Figs. 10, 11. Cardinal and profile views of a small individual, with the irregu- 

 lar growth very pronounced on both valves. 

 Upper Coal Measures. Kansas City, Missouri. 



Derbya euqincsa, Hall. 

 Page 345. 

 Figs. 12, 13, 14. Cardinal, anterior and profile views of an internal cast in 

 chert. 

 Keokuk limestone. New Providence, Indiana. 



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