34 THE GASTROPOD GENUS YVANIA 



umbilicus. There is no thickening or callosity developed upon the inner lip. 

 Some specimens seem to indicate that resorbtion of the spiral costae took 

 place within the aperture. 



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

 portion of the whorl and occupies from slightly more to slightly less than 

 one third the width of the surface between the angulation along its outer 

 border and the angulation adjacent to the suture. It is essentially flat and is 

 bounded by two low narrow angular costae. Its surface is covered by numer- 

 ous fine crescentic, transverse costae which show the form of the sinus. They 

 are low and rounded, closely though irregularly spaced, and are separated 

 by interspaces, the wider of which are flat. These crescentic costae are 

 somewhat asymmetrically curved, the greatest concavity occurring above the 

 middle of the band so that they meet the outer edge in a position somewhat 

 more advanced than their junction with the inner boundary. No revolving 

 ornamentation is present upon the slit band. 



The most conspicuous costa upon the upper surface of the whorl occurs 

 upon the angle adjacent to the suture. Below this angle and above the costa 

 which marks the inner side of the slit band occur two to four additional 

 costae. Each of the lower ones is in turn smaller than the ones above and 

 the lowest may be finer than those that bound the band, from which they 

 differ in being rounded. In a few cases the first costa below the one which 

 occupies the angle is equal to it in size. The interspaces which separate these 

 costae in general are progressively somewhat narrower between the lower 

 finer costae. The narrow, flat, horizontal area adjacent to the suture is about 

 equal to or slightly narrower than the slit band. The sloping surface between 

 the edge of the band and the angle bounding the horizontal surface along the 

 suture is about one and one half times or more often twice as wide as the 

 band. Lines of growth are well developed upon the upper surface of the 

 whorl. On the narrow flat area adjacent to the suture they bend backward 

 and meet the suture at an angle slightly acute, or they become concave and 

 develop a small sinus. The lines of growth on this portion of the shell may 

 be somewhat grouped and form rounded transverse ridges. Upon the sloping 

 portion of the upper surface the growth lines extend slightly forward from 

 the angle which parallels the suture. The most forward position is reached 

 in the upper part of this area. They then make a gently convex curve and 

 sweep strongly back, meeting the edge of the slit band at an acute angle 

 The nodose character of the costa occurring on the angle adjacent to the 

 suture, and on some specimens of the next lower one, is associated with the 

 lines of growth, but where growth lines cross the lower costae only minor 

 irregularities occur. 



The lateral surface of the whorl is about equal to or slightly wider than 

 twice the width of the slit band. It is bounded below by a prominent rounded 



