GASTEKOPODS. 187 



Keokuk and Burlington limestones are much more intimately 

 related biologically than had hitherto been generally regarded ; 

 and that many so-called Keokuk species are merely the subse- 

 quent genetic representatives of Burlington forms. The 

 validity of their distinction simply on account of occurring in 

 differently named geological horizons cannot be sustained. It 

 is not to be supposed that the biologic sequence of two divi- 

 sions, as the Burlington and Keokuk, so closely related strati- 

 graphically and lithologically, and deposited under identical, 

 quiet conditions should be so widely separated faunally as the 

 described species from these limestones would indicate. 



Genus Orthonychia HALL. 



Under Conrad's generic name, a multitude of paleozoic 

 gasteropods have been described. The genus has long been 

 made to embrace a great variety of species, some of which 

 are manifestly not at all closely related to one another geneti- 

 cally. Of these a few have been assigned lately to the groups 

 to which they more properly belong. But there still remain 

 a considerable number of shells which are clearly not mem- 

 bers of the genus, but whose generic relationship cannot at 

 present be determined with exactness. It is, however, some 

 of the more familiar species commonly referred to Platyceras 

 that are to be considered in the present connection. 



These shells fall naturally into three groups, more or less 

 easily distinguished by the general shape. One section is char- 

 acterized by having a small, closely coiled spire more or less 

 contiguous with the large campanulate body-whorl. Another 

 group includes those shells having a very small spire, usually 

 arched, but seldom closely coiled, the body-whorl being much 

 elongated vertically, or often more or less distinctly spiral. 

 A third section embraces the straight conical forms, having 

 very little or no curvature to the spiral parts. To the first of 

 these groups Montfort's generic title Capulus applies; for the 

 second and third it seems advisable to revive HalPs names, 

 Orthonychia and Igoceras. These groups may require eventu- 

 ally some further modifications, but they appear to satisfy all 



