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 



