24 THE GASTROPOD GENUS YVANIA 



The slit band is situated along the outer margin of the upper flattened 

 portion of the whorl and is equal to about one fourth of the width of this 

 surface. It is bounded by two prominent costae having narrowly rounded 

 crests and slopes which are concave toward the slit band and give this portion 

 of the shell a broadly concave appearance. On the mature portion of the 

 shell the band may be ornamented by one or more fine revolving costae. 

 No transverse markings or lines of growth have been observed upon the 

 slit band. 



A prominent costa, similar to the one which occurs upon the inner edge 

 of the slit band, lies upon the upper surface of the whorl next to the suture. 

 Between these two costae on the last whorl of the holotype occur two others 

 which are locally almost if not quite as prominent as the one adjacent to the 

 suture. No transverse ornamentation exists upon the upper slope of the 

 whorl and no lines of growth have been observed on any of the specimens 

 examined. 



The lateral slope of the whorl, below the outer edge of the slit band is 

 nearly as wide as the upper slope. It is bounded below by a prominent 

 angular or narrowly rounded costa and bears upon its surface another con- 

 spicuous costa about one third of the distance above the lower one. This 

 latter costa is steeper upon its upper slope than its lower and the space be- 

 tween it and the lower one is asymmetrically concave with the deepest portion 

 about three fourths of the way to the lower costa. The upper two thirds of 

 the lateral surface of the whorl is practically flat or only slightly concave 

 and extends smoothly from the crest of the outer bounding costa of the slit 

 band, which produces the sharp angularity of the shell, to the base of the 

 costa below. Lines of growth upon the lower third of the lateral surface 

 are nearly vertical between the costae, but above they bend sharply backward 

 to the edge of the slit band. 



The basal surface of the whorl is ornamented by numerous low, step- 

 like costae having steep outer slopes and long gentle inner slopes. The outer- 

 most of these is separated from the costa which marks the lower angle of 

 the whorl by a distance about equal to that which separates the two costae of 

 the lateral slope. Inward upon the basal slope the costae become progressively 

 more closely spaced until those upon the edge of the umbilicus are separated 

 by less than .2 mm. These costae apparently do not continue all the way 

 down into the umbilicus or else they become too faint to be discernable. 

 The holotype possesses 11 costae upon its basal surface but this number may 

 be variable and dependent to some extent upon the size of the shell. New 

 ones if added make their appearance out of the umbilical depression and 

 never by implantation. Growth lines upon the basal surface show that the 

 aperture was broadly concave in this part. 



