Flatyceras Group of l*aleozoic Gasteropods. — Key eft. 275 
in the upper Paleozoic of America, the co-extension of Flatyceras 
and Capulus was fully recognized and all the Carboniferous species 
hitherto referred to the former genus placed under the latter. At 
the same time considerable doubt was expressed as to the advisa- 
bility of associating all the forms mentioned under a single generic 
title. When it comes to extending the examination to all the 
species usually embraced in Conrads group, a very heterogeneous 
assemblage of shells is encountered. Hall, Meek and others have 
repeatedly called attention to the difficulties in the arrangement 
of the Platycerata. The first named author has even proposed 
two generic terms to include some of the more aberrant forms. 
In reviewing all the described species of the group under con- 
sideration, three more or less well defined and generally easily 
recognizable sections may be made out. The three are distin- 
guished readily from one another by their general shape ; but there 
are other features equally distinctive. One section is character- 
ized b}' having a small, closely coiled spire, more or less contigu- 
ous with a large campanulate body whorl. Another group 
includes those shells having very small apices, usually arched but 
seldom closely coiled, the last volution very much elongated ver- 
ticall)', and often spiral. The third assemblage embraces the 
straight conical forms with little or no convolution of apical 
parts. To the first of the three groups, Monfort's generic title, 
Capulus. applies; for the second and third it seems proper to re- 
vive Hall's names, Orthonjxhia and Igoceras. In a few cases 
these groups along certain lines seem to merge somewhat and 
their present limits eventually may require some modifications. 
On the whole, however, they appear to satisf}' the requirements 
much better than any of the common!}' accepted arrangements 
previously used. 
The American Platycerata now assigned to each of the three 
groups may be enumerated as follows: 
CAPULUS. 
argo Hall. auriculatus Hall. 
hilUngsi Hall. hiisenalis Hall. 
hisinvatuft Hall. buceulentxs Hall. 
ealantica Hall. carinatus Hall. 
elavatus Hall. crasnua Hall. 
eymbiura Hall. dihUatus Hall. 
dumosus Conrad. ecMnatus Hall. 
eqnilateralis Hall. erectus Hall. 
