Tlie Fauna of the MaxviUc Limestone. 381 



the plications, in their growth toward the anterior end, bend 

 decidedly toward the lateral margins. However, the mesial fold 

 is sometimes much more abrupt with a suppression of all of its 

 plications except two and in this respect the specimens resemble 

 S. keokuk. In the upper half another small form is found which 

 resembles S. keokuk somewhat more closely than it does 5^. 

 rockyniontanns. In this same division and especially in the Olive 

 Furnace Shaft occur still other forms which are larger and 

 coarser and which can with equal propriety be referred to 

 either 5". keokuk or .S. increbeseens. 



In the Maxville limestone, therefore, the separation of S. 

 keokuk and 5". increbeseens is attended with a great deal of 

 difficulty if not impracticable. The same trouble is experienced 

 in the attempt to refer some specimens to vS". rockyniontanns 

 (1858) on the one hand and to 6^. keokuk (T858) on the other. 

 That this should be the case is not at all surprising when the 

 principles of evolution are taken into consideration ; and when 

 this is done one species will probably be the result. 



Horizon and locality. — INIaxville limestone. 

 Upper zone : Mouth of Buckeye Fork, Fultonham ; Gladstone 



Mills, Thompson Residence, White Cottage. 

 Lower zone : Cut No. 4, Mt. Perry-Fultonham. 

 Undetermined zone : Harper Shaft, Olive Furnace. 



DIFLASMA TURGIDA-Hall. 



1856. Terebratula turgida. Hall, Trans. Albany Tnst., Vol. IV, p. 6. 



St. Louis limestone: Alton, Illinois; Bloomington and Spergen 

 Hill, Indiana. 



1882. Terebratula turgida. Whitfield, Bull. - Am. Mus. Nat. Hist, Vol. 



I, p. 54, pi. 6, f^gs. 53-58. 

 St. Louis group: Indiana, Iowa, Illinois and Missouri. 



1883. Terebratula turgida. Hall, Ind. Dept. Geol. Nat. Hist., r2th"Rept., 



p. 336, pi. -29, figs. 53-58. 

 St. Louis group: Alton, Illinois; Bloomington, Lanesville and 

 Spergen Hill, Indiana. 

 1891. Terebratula turgida. Whitfield, Ann. N. Y. Acad. Sci., Vol. V, 

 p. 586, pi. 13, figs. 21, 22. 

 Maxville limestone : Maxville and Newtonville, Ohio. 



