THE GASTROPOD GENUS YVAXIA 19 



having angular or narrowly rounded crests whose converging slopes give 

 to the hand a concave outline. No ornamentation is present upon the slit 

 hand. Lines of growth arc indicated by very slight unevenness of its surface 

 but their details cannot be made out. 



The two costae which border the slit band are the most prominent mark- 

 ings upon the upper sloping surface of the whorl. A third slightly less con- 

 spicuous costa parallels the suture and is separated from it by a distance 

 about equal to the width of the hand. The space between this costa and the 

 >uture is flat, smooth, and unornamented. The space between the costa and 

 the hand is slightly concave and carries three hue thread-like costa nearly 

 evenly spaced. Growth lines are not apparent upon this part of the shell. 



The lateral slope of the whorl below the outer a\v!,c of the slit hand is 

 about equal in width to the upper surface. It is hounded below by a con- 

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

 surface. This costa is broad and angular and its upper slope is somewhat 

 !>er than its lower one. Above this costa and separated from it by a 

 distar.ee of between one third to one half of the width of the lateral surface 

 is another slightly less prominent costa of similar character. The space 

 between these two costae is broadly and evenly concave and hears a single 

 fine thread-like costa. Above the second prominent costa the surface is flat 

 and smooth except for two delicate costae upon its lower part and a third 

 along the upper edge just slightly below the angulation. Lines of growth 

 are not conspicuous but can be made out here and there. They cross the 

 lower edge of the lateral surface at nearly right angles and then curve strongly 

 backward above, meeting the border of the slit hand at a very acute angle. 



The basal surface of the whorl is ornamented with numerous revolving 

 costae which are separated from each other by about .1 mm. over the greater 

 part of this surface. On the outer portion, however, they are more widely 

 spaced, the first one being separated from the costa marking the boundary 

 of the basal surface by a distance only a little less than that between the two 

 prominent costae of the lateral surface of the whorl. The second costa of 

 the base is in turn separated from the first by a somewhat smaller interval 

 and the spacing of the remainder is nearly uniform. These costae are very 

 low and are separated from each other by angular furrows which have long 

 gentle outer -lope- and short steep inner slopes. The costae of the base do 

 not extend down into the umbilicus. The holotype possesses about 15 costae 

 upon its basal surface. Growth lines have not been observed upon this por- 

 tion of the shell. 



Remarks. — The sj>ecies Yvania pusilla may he distinguished from the 

 the other similar species principally by its much smaller size. It is not im- 

 probable that the shell described above is a dwarfed form but nevertheless 

 it is sufficiently distinct from Y . inclinata and Y. gurlcyi to receive recogni- 



