1890.] The Naticoid Genus Strophostylus. 1113 
within, giving a slight indication of a columella. In all respects 
it very closely resembles the typical forms of Strophostylus cyclos- 
tomus Hall from the Niagara shales of Waldron, Indiana. The 
shells described under Platystoma subsequent to the appearance of 
the original diagnosis vary more or less from the typical species. 
The general tendency, however, has been to include under Con- 
rad’s genus those forms having a rather small, depressed spire, the 
inner lip rather thin, and simply reflected over the body-whorl. 
Often the lip does not touch the second volution and the mouth of 
the shell is frequently free for a considerable distance. Some of 
these forms closely approach Paleozoic species of Capulus. 
When, in 1859, Hall examined the interior of a Platystoma-like 
shell (which he afterwards called Strophostylus andrewsi), he 
found that the specimen had the inner lip considerably thickened 
and somewhat angular within. As a matter of course the interior 
angularity appeared twisted on account of the continued enlarge- 
ment of the shell to the adult stage. In some individuals the 
thickening was considerably exaggerated, and formed a short pro- 
jecting lamella or ridge. But from the figures given by Hall it is 
clear that this was not entirely characteristic, and the two spec- 
imens figured in the text differ widely in this particular. Never- 
theless Strophostylus was proposed and has since been applied to 
nearly a score of species. 
The limits of Conrad’s and Hall’s genera have never been pre- 
cisely defined in the descriptions of species, and the larger 
majority of the forms in question have manifestly been assigned 
to their respective groups more on account of the general shape 
of the shells than from an examination of the columellar parts, 
which were only in exceptional cases visible without the removal , 
of the hard matrix, 
From the foregoing it is evident that Strophostylus . and 
Platystoma are practically identical, and that therefore the two 
must be regarded as synonymous. The type of the first section, 
Strophostylus andrewsi, actually stands at one extremity of a 
rather extensive and variant series of shells of which Platystoma 
ventricosum is one of the median members. At the other ex- 
