GASTROPODA. 947 
Pleurotomariida. ] 
Each year the absolute necessity of extending our systematic paleontological studies 
backward as far as possible becomes more and more obvious. The rapid changes 
which took place among the early representatives of all classes of animals and the 
consequent relative ease with which the lines of evolution may be traced, gives 
them an importance in biology that is scarcely to be overestimated. For the 
Gastropoda the Lower Silurian species, because the class is but sparingly represented 
in the Cambrian, are the most likely to throw light upon the genetic relations of the 
succeeding forms, and are therefore deserving of the most careful investigation. 
Concerning the Pleurotomariide, the facts brought out in an extended study, of the 
Paleozoic species chiefly, have led us to conclusions that, while not greatly different 
from those published by Koken, who hasadopted methods more nearly in accordance 
with our own than any previous observer, are still sufficiently original to cause 
great changes in the views and nomenclature heretofore in vogue. We have gone 
into the subject more extensively than the present work demanded or perhaps even 
justified, but the knowledge gained, if it cannot all bear fruit immediately, is still 
not in vain, since it will doubtless prove of use in our future work. We have most 
carefully considered the published papers of authors who have dealt with Paleozoic 
Pleurotomariide, particularly those of De Koninck, Sandberger, Lindstrém, (hlert 
and Koken, and it would please us greatly to enter into a detailed account of their 
various views. But as this would require more space than we have at our disposal 
and would moreover be out of place in a work of this kind, we are obliged to post- 
pone it to some more fitting occasion. Incidentally, however, we shall frequently 
refer to them, especially when our opinions happen to differ. 
We believe it is admitted generally that the essential feature of the Pleuro- 
tomartide, and the peculiarity relied upon chiefly in distinguishing the family from 
other spiral shells, excepting of course the symmetrically enrolled bellerophontids 
and certain Huomphalalide, Fissurellide, Turritellide and Cerithiide, is a definitely 
limited narrow band, terminating anteriorly in the bottom of a more or less deep 
sinus of the outer lip merely, or in a long, open and sometimes periodically closed 
slit. The long parallel-edged slit occurs, as far as known to us, in but two Lower 
Silurian species (Schizolopha of this work), and is comparatively rare among the 
Upper Silurian and Devonian forms; but with the Carboniferous species it is common, 
while among the more recent forms it is nearly always present. 
Of all the characters of the family, the peculiarities of the slit and band furnish 
us with the most reliable grounds upon which to base our generic divisions. Accord- 
ing to these the family might be divided primarily into two principal groups, the 
first having a sinus only in the outer lip, the second a long parallel-sided slit or a 
series of openings. These groups are again divisible, according to the character of 
