LEPT^ENA 49 



the valves are more or less abruptly and often rectangularly deflected to 

 form a conspicuous anterior slope. The entire external surface is also 

 covered by fine, regular, radiating costa? which increase by intercalation 

 and bifurcation, and by still finer concentric markings. 



Remarks. Wherever members of this genus have been recorded they 

 have usually been referred to the single species L. rhomboidalis Wilck., and 

 while it is true that representatives of this so-called species throughout its 

 entire geologic range, resemble each other more or less closely, it is also 

 true that the individuals associated together in any one fauna do pos- 

 sess characteristics by means of which they may be distinguished from 

 any other group of individuals from another fauna. There may be a 

 difference of opinion as to whether these groups should be given specific 

 or varietal rank, but in the opinion of the writer they should be consid- 

 ered as distinct species, there being no advantage in using varietal names 

 for groups of individuals which can be clearly differentiated, and for the 

 purposes of the stratigrapher it is necessary that the several forms should 

 have some designation by which they may be referred to. Among the 

 Mississippian faunas here considered, there are at least two forms which 

 are sufficiently distinct to be easily differentiated, neither one of which, 

 it is believed, can be referred to the typical L. rhomboidalis. In Great 

 Britain the Mississippian form of Leptcena is commonly called L. analoga, 

 and one of the forms in our faunas is here referred to that species, al- 

 though it is not possible to determine from Phillip's original description 

 and figure just what the essential characters of the species are. 



LEPT/ENA ANALOGA (Phillips) 

 Plate II, Figs. 1-10 



1836. Producta analoga Phillips, Geol. Yorkshire, vol. 2, p. 215, pi. 7, 



fig. 10. 

 185*9. Strophomena rhomboidalis var. analoga Davidson, Brit. Foss. 



Brach., vol. 2, p. 119, pi. 28, figs. 1-2. 



1874. Strophomena rhomboidalis White, Prelinn. Rep. Inv. Foss., p. 17. 

 1877. Strophomena rhomboidalis White, U. S. Geog. Surveys, W. 100th 



Mer., vol. 4, p. 85, pi. 5, fig. 5. 



1877. Strophomena rhomboidalis Hall and Whitfield, U. S. Geol. Expl., 

 40th Par., vol. 4, p. 253, pi. 4, fig. 4. 



1888. Strophomena rhomboidalis Herrick, Bull. Sci. Lab. Denison Univ., 



vol. 4, pi. 9, fig. 6. 



1889. Strophomena. rhomboidalis Herrick, Am. Geol., vol. 3, pi. 4, fig. 6. 

 1892. Leptcena rhomboidalis Hall and Clarke, Int. to Study of Brach., 



pt. 1, pi. 13, fig. 9. 

 1892. Leptcena rhomboidalis Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 



pi. 8, figs. 30-31; pi. 20, fig. 24. 

 1894. Plectambonites rhomboidalis Keyes, Mo. Geol. Surv., vol. 5, p. 70. 



pi. 39, fig. 6. 



