28 THE GASTROPOD GENUS YVANIA 



by revolving costae ; slit band prominent, concave, located on the upper flat- 

 tened surface of the whorl and adjacent to the angle. 



The dimensions of the holotype are : height of the entire shell 7.4 mm. ; 

 height of last whorl 2.9 mm.; and width at base 5.8 mm. Jt is composed 

 of about seven whorls. None of the specimens in onr collections are suffic- 

 iently well preserved to determine the progressive development of the shell 

 from the embryonic state to maturity. 



The mature whorls are subrhomboidal in outline and are wider than high. 

 The outer surface is divided into three nearly equal parts by two distinct 

 angulations the upper of which is the more sharply localized and forms an 

 angle of about 130°, situated below the suture by about one third the height 

 of the whorl. The lower one, which is located below the first by about one 

 third the height of the whorl, is more in the nature of a sharp inward curva- 

 ture of the shell. The upper slope is flat and inclined slightly downward 

 from the suture. The lateral slope is nearly vertical in position and broadly 

 concave in outline. The basal surface is somewhat depressed, only gently 

 convex, and curves slightly into the umbilical region. 



The whorls are practically non-embracing, each subsequent one being 

 attached merely to the basal surface of its predecessor. The suture is sharp, 

 angular, and impressed, and follows just below the prominent costa which 

 marks the lower limit of the lateral surface of the preceding- whorls. 



The columellar lip is somewhat thickened and recurved upon itself. 

 It either extends over and completely closes the umbilicus or almost does so ; 

 which it does could not be determined satisfactorily from the specimens 

 examined. There is no thickening or callosity on the inner lip. 



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

 portion of the whorl and is equal in width to a little less than one third the 

 total width of this surface. It is bounded by two prominent costae whose 

 converging slopes give to the band a broadly concave outline. It bears no 

 ornamentation and growth lines are not visible. 



Two costae about as prominent as those which bound the slit band occur 

 upon the upper portion of the upper surface of the whorl. The inner one 

 is separated from the suture by a smooth level area a little over half as wide 

 as the band. It is this area in combination with the prominent costa above 

 which gives the suture its sharply impressed condition. The second of the 

 two costae occurs below the first and is separated by a space equal in width 

 to the area between the upper costa and the suture. The space between the 

 costa upon the inner edge of the band and the lower of the two costae de- 

 scribed above is as wide or slightly wider than the band and resembles it in 

 general appearance except that it is somewhat more concave. No growth 

 lines have been observed upon this portion of the shell. 



