249 



marck, presents one distinct and leading character 

 throughout all its species, whereby it may be easily 

 distinguished ; which is, that the widely dilated, wing- 

 shaped aperture on the right side is never dentated, or 

 digitated, as in the Pterocera; another equally remark- 

 able distinction is, that the sinus or notch at the 

 lower part of the winged aperture, is always separat- 

 ed from the canal, which is contiguous in the Ros- 

 tellaria. Shell ventricose, terminated at the base by 

 a short canal, truncated or notched ; right side dilated 

 by age, and expanded into the shape of a wing, lobed, 

 or crenulated at the upper part, with a sinus at the low- 

 er end, separated from the canal or notch at the base. 

 Other remarks may be added to the above, which will 

 perhaps assist to point out more fully the character of 

 this genus. The spire is in many species quite conceal- 

 ed by the expansion of the lip at the upper part; in 

 others it is turreted and more or less produced : in the S. 

 latissimus, the outer lip is reflected inwards at the mid- 

 dle part, either in a fold or solid plait; and in some the 

 wing is continued in a long channelled termination at 

 the upper part. The exterior of different species is 

 variously striated, smooth, wrinkled longitudinally or 

 having more or less prominent tuberculations on the up- 

 per part of the whorls, some of which present the ap- 



