THE GASTROPOD GENUS YVANIA 23 



whorl forms slightly less than half the height of the shell; surface marked 

 by numerous revolving eostae : slit hand prominent, concave, located on the 

 upper flattened surface of whorl and adjacent to the angle. 



The dimensions of the holotype which is the most completely preserved 

 specimen in our collection are: height of entire shell 4.8 mm.; height of last 

 whorl 2.2 mm.; and width at hase 4.6 mm. It is composed of about six and 

 one half whorls, ddie hrst three turns of the spire are uniformly convex 

 and smooth. Early in the course of the fourth turn the revolving eostae 

 appear, hrst upon the lateral slope adjacent to the lower suture and then 

 progressively above until four are present. The two central eostae represent 

 those which bound the slit band, the lower one is the upper large costa of the 

 lateral surface of the whorl. The uppermost costa continues prominently 

 on all the later whorls adjacent to the suture. Additional eostae originate by 

 implantation at various stages in the development of the shell. The regular 

 convex outline of the whorls persists into the fifth turn during the course 

 of which, however, the angulation of the mature portion of the shell develops 

 and the slit hand, which up to this time had been inclined, becomes more 

 nearly horizontal and takes its position upon the outer edge of the upper 

 flattened portion of the whorl. 



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

 The outer surface is divided into three parts by two angulations: the upper 

 is sharply localized forming an abrupt angle of about 130°, situated below 

 the suture by between a quarter and a third the height of the whorl; 

 the second, which is located below the first by about half the height of 

 the whorl, is less localized and more in the nature of a sharp inward curv- 

 ing of the shell. The upper slope is Hat and inclined slightly downward from 

 the suture. The lateral slope is vertical in position and flat or slightly concave 

 in outline. The basal surface is gently and rather evenly convex. The 

 convexity may or may not locally increase notabl) as the basal surface slopes 

 into the umbilicus. 



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

 whorl building downward and outward from the base of the preceding one 

 to which it is attached. The suture is sharp, angular, and slightly excavated 

 due to the conspicuous parallel costa upon the upper surface of the later 

 whorl. The suture closely follows the position of the prominent costa which 

 marks the lower angulation of the preceding whorl. 



The columellar lip is blunt and recurved upon itself but not much thicker 

 than the rc>t of the shell. It extends over and partially hides the umbilicus 

 which is >mall and shaped like a comma, being rounded at one end and drawn 

 out to a curved point at the other. There is no thickening or callosity of the 

 inner lip. Several s]>ecimens indicate that there was partial but not complete 

 resorbtion of the basal ornamentation for a short distance within the aperture. 



