TUK GASTROPOD GENUS VVANIA 21 



surface passes beneath the suture of the next whorl and disappears. Early 

 in the fourth turn the next lower costa disappears in a similar manner. 

 Later in the course of this same turn two line thread-like costae arise upon 

 the broad smooth space adjacent to the band upon the upper surface. This 

 number is subsequently increased to four. 



The mature whorls are subrhomboidal in outline and are slightly wider 

 than high. The outer surface is divided into three parts by two angulations 

 the upper of which is sharply localized along the outer border of the slit band 

 and forms an angle of about 120°, situated below the suture by about one 

 fifth the height of the whorl. The lower angulation is little more than a 

 regular inward curving of the shell from the flat lateral surface to the convex 

 basal surface of the whorl. The lateral surface constitutes a little more than 

 half the height of the whorl and is practically vertical in position. The slope 

 of the basal surface is gently and nearly uniformly convex into the umbilicus. 



The whorls are practically non-embracing, as the spire is formed by each 

 succeeding whorl being attached to little more than the basal surface of the 

 preceding one. The suture is almost right angular and either follows or 

 occurs slightly above the prominent costa which marks the beginning of the 

 curvature into the basal surface of the preceding whorl. 



The columellar lip is blunt and recurved upon itself but not greatly 

 thickened. It extends over and partially hides a moderately sized umbilicus. 

 Xo thickening or callosity is apparent upon the inner lip. 



The slit band is situated along the outer margin of the upper sloping por- 

 tion of the whorl and is inclined slightly downward from its general direction. 

 The band is equal in width to slightly more than one fifth of the complete 

 width of the upper surface of the whorl and is bounded by two prominent 

 costae having angular crests whose converging slopes give to the band a con- 

 cave outline. Xo ornamentation is present upon its surface. Lines of growth, 

 however, are faintly indicated by slightly concave transverse markings which 

 meet the outer of the bounding costae at a position a little farther to the front 

 than that at which they touch the inner. 



The two costae bordering the slit band and a third costa adjacent to the 

 suture are the most prominent markings upon the upper sloping surface 

 of the whorl. The costa last mentioned is separated from the suture by a 

 flat and nearly horizontal area a little wider than the width of the slit band. 

 Between the upper prominent costa and the slit band the surface is shallowly 

 concave and carries as many as four fine thread-like costae which are some- 

 what more pronounced and closely spaced upon the inner upper portion. Lines 

 growth are well developed upon the upper surface of the whorl and arc 

 displayed by fine lirae and unevenness of the shell surface. They are linguli- 

 form and attain their farthest forward position about midway between the 



