184 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
ON THE MACROCHEILUS OF PHILLIPS, PLECTOSTYLUS OF CON- 
RAD, AND SOLENISCUS OF MEEK & WORTHEN. 
By CHARLES A. WHITE. 
Among the somewhat numerous species of fossil shells which have 
been published from North American Devonian and Carboniferous rocks, 
especially the latter, and which have been referred to the genus Macro- 
cheilus of Phillips, are certain forms which plainly do not answer the de- 
scription of that genus as it was originally given or as its characteristics 
have usually been stated by authors. The differences between these spe- 
cies and those which I regard as true Macrocheilus pertain mainly to the 
columella and inner lip, but they also possess a more massive test than 
those which are referred to Macrocheilus proper. Some of the American 
species which have been referred to Macrocheilus have a plain, more or 
less sinuous inner lip, which is only slightly covered with callus, and 
destitute of any trace of ridges or folds. These I assume to be typical 
forms of that genus, and the following, among others, may be mentioned 
as examples: Macrocheilus hebe and M. hamiltonie Hall, of the Devonian, 
and M. anguliferus White, of the Carboniferous. My present belief is 
that all the Devonian forms which have been referred to Macrocheilus 
will fall into this group, but it will properly include only a very small 
part of those which have been referred to that genus from Carboniferous 
strata. 
With the very few exceptions referred to, I think that all the numer- 
ous North American Carboniferous forms which various authors have 
referred to Macrocheilus constitute a distinct natural group, which ought 
to be designated by one and a different generic name. I also think the 
form for which Meek and Worthen proposed the generic name of Solenis- 
cus should be included in this group. 
The shells of this group are characterized by a more or less thickened 
inner lip, which also bears one more or less distinct revolving fold. This 
fold, when the outer lip is entire, is usually visible only as an obtuse 
prominence near the anterior end of the inner lip, but upon breaking 
away the outer lip the fold is usually found to be distinct and often sharp 
and prominent. Sometimes also there is upon the posterior side of the 
fold a broad, concave depression, which ends at and deepens the inward 
flexure of the inner lip, the posterior border of which depression is some- 
times so well defined as to appear like a second revolving fold. This 
depression, which is sometimes a tolerably distinct groove, is excavated 
out of the callus which covers the columella and inner lip, in such cases 
quite thickly, between the depression and the posterior angle of the 
