938 THE PALEONTOLOGY OF MINNESOTA. 



[Baphtetomidffi. 



of the whorls consequently different; finally, there is always an interruption or 

 peculiar bend in the backward sweep of the surface striae on the flat upper side of 

 the whorls, the like of which we have never seen in any Eccyliopterus. The last 

 peculiarity we have observed in only three other groups of species that have been 

 referred to the Pleurotomariidce, namely, the Upper Silurian Euomphalopterus, 

 Roemer, a good genus that will include the majority if not all of Lindstrom's section 

 alatce of Pleurotomaria; our Raphistomina, founded upon Lower Silurian shells of the 

 type of Rctphistoma lapicida Salter; and our Omospira, which differs from Raphistoma 

 chiefly in forming a high spire and in having much less angular whorls. 



For the present we must take a decided stand against the view held by many 

 paleontologists and definitely expressed by Koken (op. cit., p. 315) that the develop- 

 mental series in which Raphistoma was continued ended in unquestionable pleuroto- 

 marians. On the contrary so far as our observation permits of judgment, Raphistoma 

 is a sharply limited type, in no case taking on pleurotomarian characters, and one 

 that is entirely restricted in its geological range to the rocks lying beneath the base 

 of the Upper Silurian. As may be seen from our R. richmondense, which is from 

 the uppermost division of the Cincinnati period and the most recent species of the 

 genus known, the generic type underwent exceedingly little modification from 

 the first to the last. Nor have the changes been in any respect toward the 

 Pleurotomariidd;. We admit, however, that a more complete knowledge of the 

 Calciferous Gastropoda is necessary before it will be possible to reach perfectly 

 satisfactory conclusions respecting the origin and development of Raphistomina and 

 Raphistoma and their true relations to the equally ancient euomphaloid and 

 pleurotomarian genera Eccyliopterus and Helicotoma, and Liospira, Euconia, Eotomaria 

 and Lophospira. All these genera represent, in a measure, contemporaneous lines 

 of development, often exhibiting very nearly parallel, or at any rate similar, series 

 of modifications. But this does not necessarily imply that they pass into each other, 

 not that there is any very close relationship amongst them. They may have merely 

 given expression to characteristics and tendencies which they inherited in common 

 from a remote ancestor. 



If genetic relations exist between the R<tphistomid<e and Pleurotomariidd', and 

 this is a condition that we believe will some day be demonstrated, then the faint 

 sinus in the outer part of the upper lip may prove to be an undeveloped or incipient 

 representation of the much narrower and deeper apertural notch of the early 

 pleurotomarians. In that case Koken's view of the carina of Euomphalopterus 

 (op. cit., page 318) which he gives in opposition to Lindstrora, who regards it as 

 homologous with the slit-band of the Pleurotomariidd', would be correct in so far as 

 the shells are concerned which we regard as Pleurotomariidd'. But he is certainly 



