TIIK GASTROPOD GENUS YVANIA 17 



three thread-like costae. Growth lines are fairly well shown upon the lateral 

 surface of the whorl. They meet the costa which marks the lower limit of 

 this portion of the shell at right angles and above curve strongly backward, 

 approaching the border of the slit hand at a very acute angle. 



The basal surface of the whorl is ornamented by numerous revolving 

 costae which are separated from each other by about .15 mm. over the greater 

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

 spaced, the first two being separated from each other and the costa marking 

 the boundary of the basal surface by distances about equal to that which 

 intervenes between the two most prominent costae of the lateral surface of 

 the whorl. A very tine thread-like costa is present in the outer of these 

 interspaces. The costae of the base apparently do not extend down into the 

 umbilicus. The holotype possesses 15 costae upon its basal surface. Growth 

 lines are not conspicuous upon the base but where oberved are broadly and 

 shallowly concave over most of this portion of the shell. 



Remarks. — The species Yvania inclinata is closely related to Y. pusilla 

 and Y. gurleyi. The 1". pusilla, which is probably a dwarfed form, may be 

 di>tinguishcd from I*, inclinata not only by its much smaller size but also by 

 its relatively narrower slit band and the outward slope of the lateral surface 

 of its last whorl. The ornamentation of revolving costae is also somewhat 

 more simple. Yvania inclinata may be distinguished from Y. gurlcyi by its 

 wider slit band bounded by more prominent costae, by the finer nature of its 

 other markings, and by the thread-like costae adjacent to the suture and 

 upon the lateral surface of the whorl. 



Yvania inclinata is associated with Y. supercrenata from which it may 

 be differentiated by the concave upper surface of the whorl, the slightly in- 

 clined and narrower slit band, the lower spire, the finer and more numerous 

 revolving costae. and the more uniform spacing of these costae upon the base. 



The holotype of this species i^ Xo. 19 in the collection of the Illinois 

 State Geological Survey. 



Occurrence. — This species occurs in a limestone bed which closely under- 

 lies the Colchester (Xo. 2) coal over large areas in western Illinois. The 

 holotype and a series of other si/>ecimens were collected from an outcrop in 

 the northeast bank of Wilson Creek near the southwest corner sec. 26, 

 T. 3 X., R. 2 E.. Fulton County, Illinois. 



Yvania pusilla Weller, n. sp. 

 PI. I, fius. 4a-c 



Description. — Shell very small, somewhat wider than high, slightly tur- 

 reted. spiral angle about 96° ; whorls angular, base convex, umbilicus mod- 

 erate; last whorl forms over half the height of the shell; surface marked 



