186 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
figure appears to represent the outer lip as unbroken; but to exhibit 
the columellar fold so prominently as that figure shows it to be, the 
outer lip must have been largely removed. 
Understanding the real characters of the type-species of Solengeoun’ to 
be such as I have here indicated, it is, I think, necessary to regard it as 
congeneric with the greater part, if not all, of ee forms which are fig- 
ured with it on Plate VIII, and with mee of those Carboniferous shells 
which have been by different authors referred to Macrocheilus. The prin- 
cipal differences, according to my observations, which that species pre- 
sents from the others referred to, are its more than usually elongate form, 
a little greater than the usual prominence of the anterior part of the 
aperture, and a smaller accumulation of callus upon the inner lip. 
These forms, as before remarked, are regarded as constituting a nat- 
ural group, which, it appears to me, well deserve a generic designation 
distinct from Macrocheilus. If it were not that Conrad’s name, Plecto- 
Stylus, was preoccupied by Beck, that name would be appropriately re- 
tained for this group, to which it was really applied. Conrad’s name 
not being available, the next generic name that has been used for any 
member of the group ought to be used for the whole group. As Solen- 
iscus is regarded as a member of this group, that name should be prop- 
erly used for it because no other available name has priority. 
The following species which have been hitherto referred to Maero- 
cheilus have been found to possess the prominent columellar fold and 
other characteristics of the group here discussed, and I would therefore 
refer them to Soleniscus: Macrocheilus fusiformis Hall, M. newberryi 
Hall, MW. planus White, MW. ventricosus Hall (= Soleniscus brevis White), 
M. texanus Shumard? M. paludineformis Hall, and M. halli Geinitz. 
All except the last are figured on Plate VIII. 
It is not to be denied that there are certain forms among those Car- 
boniferous species which have usually been referred to Macrocheilus 
that possess at best only an obtuse fold upon the columella. They are, 
however, much more closely related by all their characteristics to the 
species just referred to Soleniscus than are those Devonian and other 
species which I have referred to Macrocheilus proper. Among these 
species are the three following, which are represented with the others 
on Plate VIII: Macrocheilus ponderosus Swallow? M. medialis Meek & 
Worthen, and M. primigenius Conrad. These I regard as at best no 
more than subgenerically different from those which I refer to Solenis- 
cus. 
As to the family relations of the shells of this group I am inclined to 
adopt the views suggested by Meek, that they belong to the Actawonide. 
Those shells which I have referred to the genus Macrocheilus proper are 
perhaps not suggestive of such a relationship, but they do not appear 
to differ from the Soleniscus group any more than some recognized gen- 
era of the Actconide do from certain other genera of that family. 
