1116 The American Naturahst. [December, 
stages to S. (Platystoma) ventricosus. In some of the varietal 
examples of the last-mentioned species a peculiar change in the 
inner lip begins to make its appearance. But this feature is per- 
haps better shown in a large series of S. niagarensis,its near con- 
gener. The lip, instead of being firmly united and fused with 
the body-whorl, as is the case with most of the forms originally 
described under the genus, commences to separate from the ad- 
joining volution. In these shells the columellar parts become 
greatly reduced, and even almost wholly reduced, while the 
thickening of the inner lip entirely disappears. Nor does the 
change in this feature end here. Many of the individuals show a 
tendency to uncoil, some of them having the last whorl separated 
from the preceding volution by more than half a turn (Pl. XX XIII, 
Fig. 3) The resemblance of certain varieties to some forms of 
Capulus (Platyceras of Conrad) is very striking, as has been re- 
marked elsewhere. 
The genetic relationships, as at present understood, of the lead- 
ing species of Strophostylus are graphically represented in the 
accompanying scheme (Pl. XXXIIL). The earliest-known forms 
of this group are from the Niagara rocks; but the extended ver- 
tical range of such species as S. ventricosus would indicate that 
the specific type has a higher antiquity than present information 
would suggest. Three principal series developed from this primi- 
tive type: (A) one preserving more or less distinctly the original 
characters ; (B) another degenerated more or less, giving rise to 
loosely coiled shells and those approaching the Capulus group; 
and (D) a third exhibits intensified features which are particularly 
noticeable in the region of the columella. Fig. 1 represents the 
species upon which Conrad based his genus Platystoma, and was 
called P, ventricosum. It chances to be the most generalized as 
well as one of the oldest forms of the group. The series D 
Showed a continued progression in the development of the axial 
parts, and finally ended in a form having a conspicuously twisted 
columella, as was acquired by S. andrewsi (Fig. 23). This exag- 
gerated character in the species last alluded to was the basis of 
Hall’s genus Strophostylus. But it will be seen at once that the 
species selected was actually an extreme development of a variant 
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