GASTEROPODA. 
101 
This shell can usually be readily distinguished by its widely expanded outer 
volution, the broad, shallow sinuosity in the anterior margin of the lip, and 
the absence of all markings on this part of the shell except striae of growth. 
The prominent posterior part of the volution has more or less strongly 
defined costae parallel to the striae of growth. In some specimens this is 
scarcely a conspicuous feature; in others it is limited to so small a portion of 
the volution as hardly to be seen when looking directly upon the surface, as 
the fossil is usually imbedded in the rock with the mouth downward. There 
is also great variation in the size of these costae, which in some specimens are 
twice as strong as in others, and the spaces between them two or three times 
as wide as the elevated portions; while in other examples they are reduced to 
regular equal striae: and in all cases they gradually become obsolete by 
merging into the ordinary surface striae. These phases are illustrated in 
figures 17-23. In most of the specimens examined, the expanded portion of 
the volution has been broken off, and there remains only the strongly marked 
portion of the surface. The removal of the shell leaves a smooth cast of the 
interior. 
In all the specimens determined, the margin of the lip is found to be slightly 
recurved. The expanded margin of the aperture extends around the posterior 
part of the volution, nearly inclosing it, as shown in figures 17-19; and more 
especially on looking upon the interior, as shown in figures 27 and 28, where 
the margins are nearly conjoined on the posterior side. This expanded lip is 
usually recurved and thickened in the umbilical region, the heavy callus 
extending more or less completely over the posterior part of the volution. At 
the sides this callus is smooth, but in the central portions it is elevated 
and pustulose, following the convexity of the volution, and gradually extended 
outward from the aperture and forward on the volution. It sometimes nearly 
closes across the area, but usually leaves a free space or sinus where the 
transverse costae remain visible, while the pustulose callus extends forward on 
each side, as shown in figures 22, 23, 27-29, and also in figures 5 and 6 of 
plate 24. 
In many examples, the pustules upon the callus appear to have no regular 
