IOWA ACADEMY OF SCIENCES. 25 



EVOLL'TION OF STROPHOSTYLUS. 



BT CHARLES R. KEYES. 



ABSTRACT.* 



Recently a larg^e series of ot the most important species of Platystoma and Stro- 

 phostylus was examined and the matrix carefully removed from the apertural por- 

 tions of many of the shells. The structural features disclosed in the various forms 

 show a relationship between the two established genera that was long suspected. 

 The two types are now regarded as identical. 



The genetic relationship, as at present understood, cf the leading species of Stro- 

 phostylus are graphically represented in the accompanying scheme. The earliest 

 known forms of this group are from the Niagara rocks; but the extended vertical 

 range of such species as 6". veniricosiis would indicate that the specific type had a 

 higher antiquity than present information would suggest. Three principal series 

 developed from this primitive form: (1) one preserved more or less distinctly the 

 original characters: (2) another degenerated more or less, giving ri?e to loosely 

 coiled shells and those approaching the Copulus group; and (3) a third acquired 

 intensified features, which are particularly noticeable in the region of the columella. 



At the base is the species upon "which Conrad founded his genus Platystoma, and 

 was called P. rejitricosum. It chances to be the most generalized as well as one ot the 

 oldest forms of the group. The (o) third series shows a continued progression in the 

 development of the axial parts, and finally ended in a form having a conspicuously 

 twisted columella, as was acquired by 6'. nudreivsi. This exaggerated character in 

 the species last alluded to was the basis of Hall's genus Strophostylus. But it will 

 be seen at once that the species selected was actually an extreme development of a 

 variant series, and is connected by a complete gradation of forms with the earlier 

 and less specialized one. Later in the history of the most primitive form now 

 known an exceedingly variable series was given off, which assumed in the several 

 species diverse characters. Some vary towards the 5. amh-eicsi type, while others 

 tend towards the S. niar/arensis section. In the variable forms of i>. ttirbinatiis 

 some significaut phases are represented, which suggest the relationship of these shells 

 to certain other genera. In the extreme form appears an elevation of the spine, 

 that is unknown elsewhere in the group. Some examples show scarcely any thick- 

 ening of the inner lip or columella, while others have these features well de- 

 veloped. 



It must be borne in mind that the scheme as here represented is intended to 

 indicate merely the lines along which the several developments took place, rather 



♦Published in full with one plate in the American Naturalist. Vol. XXIV. pp. 1111- 

 1117. pi. x.x.xiii. December, 1890. 



