18 AMERICAN JOURNAL 



This shell resembles most ^S'. Keokuk, Hall, of the Carboni- 

 ferous of Iowa, (See Geol. Rep. Iowa, part 2, pi. 20, fig. 3, a, 

 b.) but in that species the mesial fold of the ventral valve is 

 composed of two large ribs. The beak of the dorsal valve is 

 not so distinctly incurved in the present species ; the area is 

 shorter, broader and higher, and the sides of its dorsal valve, 

 from the beak to the angles, slope more rapidly than in /S'. Keo- 

 kuk. The mesial sinus is also more prolonged at the base than 

 in the Iowa shell. The resemblance is rather one of outline 

 than of detail. 



Index to Plates. 



Plate 3, fig. 1, la. Ammonites Kevadanus, Gabb. 

 Tu7'ho elevatus, Gabb. 

 9a. Cardium arcceformis, Gabb. 

 Ammonites Colfaxii, Gabb. 

 Septum of Am. BillingsianuSy Gabb. 

 Turbo regius, Gabb. 



6. PhoJadomya midtilineata, Gabb. 



7. P. Kevadana, Gabb.' 



10, 10a. Astarte apprcssa, Gabb. 



8. Croniomya aperta, Gabb. 



11, 11a. Cardinia ponderosa, Gabb. 



12, Posidonomya JBlatchleyi, Gabb. 



15, 15a. Plicatida perimhricata, Gabb. 



13, 13a, 13b. Cassianella lingulata, Gabb. 



14, 14a. Monotis circularis, Gabb. 



16, IGa, 16b. Spirifer obtusus, Gabb. 



