GASTROPODA. 881 
Tetranota wisconsinensis.] 
TrETRANOTA WIScoNsINENSIS Whitfield. 
PLATE LXV, FIGS, 26—29. 
Bellerophon wisconsinensis WHITFIELD, 1878, Ann. Rept. Geol. Surv. Wis., p. 76; also 1882, Geol. Surv. 
Wis., vol. iv, p. 228, pl. vi, figs. 15, 16. 
Greatest hight of shell varying in the material before us from 8 mm. to 37 mm.; 
closely coiled for the genus, subglobular in form when young, but becoming strongly 
bilobed, with the last volution less inrolled and the lips greatly expanded laterally 
and somewhat also above (yet not materially thickened) in the adult form; in the 
latter the outer lip is deeply notched in the middle, the notch spreading anteriorly 
more slowly than usual, the lobes on each side rounded-triangular in outline. 
Dorsal periphery of casts with a broad, revolving band, the top of which may be 
slightly convex, flat or concave, the whole increasing in elevation with age; on each 
side of the central band the inner volutions (as seen in fractured specimens) have 
first a concave space and then a ridge, but these pertain chiefly to the exterior side 
of the shell, being only in rare cases, and always with difficulty, distinguishable on 
casts; inner volutions elongate-reniform in section, outer volution more triangular; 
umbilicus comparatively small, the width only about one-fifth of the greatest 
diameter of the shell. Surface markings not fully determined, apparently as in 7. 
obsoleta and T’. bidorsata. 
The essential generic characters (i. e., the revolving ridges) were overlooked by 
Prof. Whitfield. Still he noticed the resemblance to the adult form of 7. bidorsata, 
which species he regarded as a Bucania, while his wisconsinensis he held to be “a 
true Bellerophon, as is readily seen by the closed or nearly closed umbilicus.” But 
differences in the size of the umbilicus, when other characters agree, are now con- 
sidered as of small importance. In this case certainly it is not of material 
consequence. Besides, the umbilicus is not by any means closed, there having 
been no axial thickening which might have filled the not very small cavity seen in 
casts. 
Young specimens showing the inner volutions of T. wisconsinensis closely resemble 
those of T. obsoleta, the only difference observable in casts being that the umbilicus 
is slightly smaller and the volutions more ventricose in the present form. Adult 
specimens of the two species cannot be confounded, the last volution being higher, 
with flattened slopes, and less inrolled in 7. wisconsinensis, the whole producing a 
very differently shaped aperture. 7’. bidorsata and T. sexcarinata have more com- 
pressed whorls, much larger umbilicus, and a wider as well as otherwise differently 
shaped mouth, while for both the lateral ridges are conspicuous features of casts, 
which is not at all the case in 7. wisconsinensis. 
—56 
