1890.] The Naticoid Genus Strophostylus. 1115 
Strophostylus, as now understood, embraces three rather well- 
marked types of shells. One of these sections contains chiefly 
those extreme forms upon which the genus was originally 
founded. These shells are subglobose, with the spire somewhat 
elevated; the columellar parts are prominent, and the front por- 
tion of the inner lip is considerably thickened, often having a 
distinct depression or groove which continues inward around the 
columella. This group finds its greatest development in the Up- 
per Silurian. Another section includes shells similar to Strophos- 
tylus (Platystoma) miagarensis, in which the spire is depressed, the 
inner lip simply anchylosed to the body-whorl, and thickened to 
little or no extent. These forms predominate in the Devonian. 
They closely approach certain Capuli which have been called 
Platycerata, and it is very probable that the generic position of a 
number of species in the latter genus will be modified upon more 
critical examinations of all the forms. To the third section be- 
long chiefly Carboniferous shells like Strophostylus (Platystoma) 
peoriensis McC. 
The most striking points of difference in the various forms 
here referred to Strophostylus are found in the columellar region 
and along the inner margin of the aperture. The columella is in 
no species perforated for the umbilicus. In those species showing 
these parts in the greatest perfection there isa considerable thick- 
ening of the forward portion of the inner lip. This thickening is 
frequently more or less distinctly grooved within, parallel to the 
apertural margin, and consequently a prominent ridge often bor- 
ders the groove towards the interior. As the shell increased in 
size the inner lip became reflexed, making the parts still more 
massive. The spiral ridge around the axis of the shell thus ac- 
quires considerable prominence in sóme places. In the extreme 
forms, as Strophostylus andrewsi,—the species upon which Hall 
based his genus,—the columellar ridge forms a kind of elevated 
lamella, but this exaggeration is by no means constant in the dif- 
ferent shells of the species, and in some specimens it is scarcely 
More conspicuous than in certain individuals of typical Strophos- 
tylus matheri, or even examples of S. eyelostomus. Through the 
latter two closely allied forms the transitions are made in easy 
