26 THE GASTROPOD GENUS YVANIA 



a centrally located, fine thread-like costa which gradually becomes stronger. 

 The later whorls of some of the larger specimens are enlarged at a rate which 

 is somewhat less rapid than that of the younger whorls and the result is a 

 spire having a slightly convex profile. 



The mature whorls are subrhomboidal in outline and are somewhat 

 wider than high. The outer surface is divided into three parts by two angu- 

 lations the upper of which is sharply localized and forms an abrupt angle of 

 about 125°, situated below the suture by about one third the height of the 

 whorl. The second, which is located below the first by a little less than one 

 half the height of the whorl, is less localized and more in the nature of a 

 sharp inward curving of the shell. The upper slope is flat and inclined slightly 

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

 slightly concave in outline. The basal surface is gently convex, curving into 

 the columella. 



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 distinct and occurs at or near the 

 position of the prominent costa which marks the lower angulation of the 

 preceding whorl. 



The columellar lip is somewhat thickened and recurved and completely 

 or nearly closes the umbilicus. 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 to one third of the entire width of this 

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

 crests whose converging slopes give the band a broadly concave profile. The 

 band is unornamented by spiral markings and lines of growth are rarely 

 preserved upon it. Where they do remain they are seen to be unevenly 

 crescentic with the axis of the sinus located above the middle of the band so 

 that the growth lines meet the outer costa at a position a little in advance of 

 their junction with the inner one. 



The flat upper surface of the whorl bears two costae between the suture 

 and the costa which lies upon the inner side of the slit band. These may be 

 equally prominent or the lower one may be less conspicuously developed, 

 but never are they quite as pronounced as those which bound the slit band. 

 The spacing of the upper costae may be equal or not ; where it is not the 

 lower costa is usually much the smaller. Lines of growth are rarely to be 

 observed but where present they show that this portion of the aperture was 

 convex and linguliform and curved strongly backward toward both the suture 

 and the slit band. 



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

 nearly as wide as the entire upper slope. It is bounded below by the begin- 



