380 Proceedings of the Ohio State Academy of Science. 



he would have adopted Whitfield's and added S. increhescens 

 to the latter's list. 



Hall's inability to separate 5. keokuk var. (St. Louis lime- 

 stone, 1858) from 6". keokuk (Keokuk limestone, 1858) seems 

 especially significant. S. keokuk appears to represent one ex- 

 treme, the short hinge line, and .S". increhescens (Kaskaskia lime- 

 stone, 1858) the other extreme, the long hinge line, of an other- 

 wise very similar series of specimens. Between the two is 

 6". keokuk var. with specimens exhibiting both long and short 

 hinge lines. The ones with the long hinge lines might as well 

 have been selected as the type in this case and the name 5". 

 increhescens var. applied rather than 5^. keokuk var. To be 

 sure, it is not difficult to identify specimens of one extreme as 

 S. keokuk and those of the other as 6^. increhescens, but what 

 is to be clone with the intermediate forms which fall as readily 

 under one species as "the other? 



Just why Spirifer opimus (Coal Measures of Ohio, Mary- 

 land and Iowa, 1858) should have been originaly referred to 

 5. rockyuiontani (Moutain limestone of New Mexico, 1858) 

 rather than to 5^. keokuk, which it more closely resembles, is not 

 clear. .S. opimus is undoubtedly an oft'spring of one of the 

 Mississippian forms, and more probably one from the adjacent 

 Mississippian series rather than one from the western Mis- 

 sissippian. 



If any specific division is to be made in the above forms 

 it would seem to fall between 6". rocky-montani (1858) on the 

 one hand and 5". keokuk (1858), S. keokuk var. (1858), 5. 

 increhescens (1858), and 6". opimus (1858) on the other. This, 

 of course, would give specific rank to the first and second and 

 make all of the rest synonyms of the second. Should no divi- 

 sion be deemed advisable then all of Hall's species become syn- 

 onyms of Spirifer rockyniontanus Marcou (1858). 



In the lower half of the IMaxville limestone are found speci- 

 mens of Spirifer which resemble very closely the figures of 6^. 



rockymontanus Marcou (especially 4 c-e) in two respects: 

 first, the plications are small and sharply angular ; and second. 



