THE GASTROPOD GENI - 1\ WIA 33 



live equall) prominent and uniforml) spaced spiral costae arc apparent. The 

 slit band occupies the space above the central costa. Early in the fourth 



turn the slit band becomes proportionately wider and more prominent and 

 the angulation along its outer border begins to become apparent. Also the 

 lowest of the costae mentioned above disappears beneath the suture. < )n the 

 latter part of the fourth turn a tine thread-like costa appears by implanta- 

 tion in the interspace above the slit hand and another line costa is developed 

 below the band. In the course of the fifth turn the whorl becomes essentially 

 mature in all its characters. The costa adjacent to the suture becomes quite 

 prominent and marks a distinct angle between the sloping upper surface of 

 the whorl and the narrow level shelf along the suture. This costa, and some- 

 times the next lower one. becomes nodose. Additional costae are introduced 

 within the area between the nodose costa and the slit band, as they also are 

 upon the lateral slope of the whorl below the slit band. The last whorl of 

 the large specimen mentioned above shows gerontic development, chiefly 

 evinced by the costae of the upper slope of the whorl becoming obsolete and 



rowth lines becoming much more rugged and conspicuous. In addition 

 to this change in ornamentation the slit band becomes slightly concave. 



The mature whorls are subrhomboidal in outline and are wider than 

 they are high. The outer surface is divided into three parts by two angula- 

 tions in addition to which a third angulation occurs along the upper border of 

 the whorl adjacent to the suture. The upper of the two main angulations 

 is situated along the outer edge of the slit band and forms an angle of about 

 135 . This angulation occurs below the suture by somewhat more than one 

 third the height of the whorl. The second angulation is located below 

 the first by slightly less than one third the height of the whorl. I: is not 

 a- sharp as the upper one but marks the abrupt inward curve of the basal 

 surface. The upper surface is rather sharply concave just above the slit 

 band. From this depression the surface slopes outward to the margin of 

 the slit band and upward to the angulation which occurs adjacent to the 

 suture. This latter angulation is one of about \2? and is separated from 

 the suture by a narrow horizontal area. The lateral surface of the whorl 



July concave. In mosl cases it slopes inward a little due to the greatest 

 diameter of the shell being developed along the edge of the -lit band. The 

 basal surface is regularly convex and slopes evenly into the columellar area. 



The whorls are non-embracing, each one being attached to the base of 

 it- predecessor. The suture is sharp, angular, and may be slightly excavated. 

 It closely follows the COSta which marks the lower main angulation of the 

 preceding whorl. 



The columellar lip is strongly recurved upon itself ami considerably 

 thickened. It extends over and in most specimens completely closes the 



