18 THE GASTROPOD GENUS YVANIA 



by numerous revolving costae ; slit band prominent, concave, located on upper 

 sloping surface of whorl and adjacent to the angle. 



The dimensions of the holotype which is the largest specimen collected 

 are : height of entire shell 3.3 mm. ; height of last volution 2.0 mm. ; width 

 at base 3.6 mm. This specimen is composed of five whorls. The first three 

 turns are uniformly convex. A number of fine revolving costae are present 

 upon the surface of the third turn but they are so faint that nothing can be 

 said concerning their number or arrangement. At the beginning of the fourth 

 turn there are plainly developed four costae of about equal prominence and 

 nearly equally spaced, the second and third of which bound the area of the 

 slit band. By the end of this turn the whorl has achieved its mature angu- 

 lation. The slit band has become very little wider than it was at the begin- 

 ning of this turn but the other costae have spread apart to accommodate 

 themselves to the enlarged surface of the whorl, and a fine thread-like costa 

 has made its appearance in the space just above the slit band. In the course 

 of the fifth and last whorl two more fine costae appear in the space above 

 the slit band and in a similar manner three appear upon the flattened space 

 below it. 



The mature whorl is subrhomboidal in outline and about as wide as 

 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 125°, situated below the suture by about one 

 fourth the height of the whorl. The lower angulation is not so sharp 

 but consists of an abrupt inward curving of the shell from the nearly flat 

 lateral surface to the convex basal surface of the whorl. The lateral surface 

 makes up a little more than one third the height of the whorl, it is slightly 

 concave and slopes outward from a vertical position inasmuch as the diameter 

 of the whorl at the lower angulation is somewhat greater than at the upper. 

 The curve of the basal surface is gently and nearly uniformly convex until 

 the umbilicus is approached into which it plunges more steeply. 



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

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

 one. The suture is narrowly rounded and occurs at or near the prominent 

 costa which marks the beginning of the curvature into the basal surface of 

 the preceding whorl. 



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

 greatly thickened. It extends over and partially hides the umbilicus. No 

 thickening or callosity is present upon the inner lip. 



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

 portion of the whorl and is inclined slightly downward from its general 

 direction. The band is equal in width to about one fifth of the complete width 

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



