938 THE PALEONTOLOGY OF MINNESOTA. 
[Raphistomids, 
of the whorls consequently different; finally, there is always an interruption or 
peculiar bend in the backward sweep of the surface strie on the flat upper side of 
the whorls, the like of which we have never seen in any Lecyliopterus. The last 
peculiarity we have observed in only three other groups of species that have been 
referred to the Pleurotomariide, namely, the Upper Silurian Euomphalopterus, 
Roemer, a good genus that will include the majority if not all of Lindstrém’s section 
alate of Pleurotomaria; our Raphistomina, founded upon Lower Silurian shells of the 
type of Raphistoma 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 I. vichmondense, 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 
Pleurotomariide. 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 Hccyliopterus and Helicotoma, and Liospira, Huconia, 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 Raphistomide and Pleurotomartide, and 
this is a condition that we believe will some day be demonstrated, then the faint 
sinus 1n 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 Huomphalopterus 
(op. cit.. page 318) which he gives in opposition to Lindstrém, who regards it as 
homologous with the slit-band of the Plewrotomariide, would be correct in so far as 
the shells are concerned which we regard as Pleurotomariida. But he is certainly 
