30 THE GASTROPOD GENUS YVANIA 



Yvania adamsi (Worthen) 

 PI. II, figs. l-3b; PI. Ill, fig. 4 



1884 Plcurotomaria adamsi. Worthen, Illinois State Mus. Nat. Hist., Bull. 2, 

 p. 5. 

 Coal Aleasures : Peoria County, Illinois. 



1890 Plcurotomaria adamsi. Worthen, Geol. Survey of Illinois, vol. 8, p. 137, 

 PI. 23, figs. 5-5a. 



Coal Measures : Peoria County, Illinois. 



Description. — Shell small, a little higher than wide, trochiform, spiral 

 angle about 65° ; whorls angular, base nearly flat, umbilicus small ; last whorl 

 forms somewhat over one third the height of the shell ; surface marked by 

 revolving costae ; slit band prominent, concave, located on upper sloping- 

 surface of the whorl and adjacent to the angle. 



Idie dimensions of an average specimen are : height of the entire shell 

 7.1 mm. ; height of the last whorl 2.8 mm. ; and width at base 6.2 mm. About 

 seven whorls are present in mature individuals. The first two and one half 

 whorls are uniformly convex. Two revolving costae become apparent at 

 about this stage in the development and are located a little above the periphery 

 of these rounded whorls. These costae inclose the area of the slit band. 

 They gradually become more prominent and during the course of the fourth 

 turn the shell takes on its mature form and ornamentation. 



The mature whorls are subrhomboidal in outline and are wider than 

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

 tions, the upper of which is sharply localized along the costa which marks 

 the outer edge of the slit band. The angle of this edge is slightly acute but 

 due to the slight concavity of the band and the greater concavity of the lateral 

 surface of the whorl these two sides of the whorl form an angle of about 

 130°. This angle is situated below the suture by slightly less than one third 

 the height of the whorl ; the lower angle is located below the first by a little 

 less than half of the height of the whorl. The upper surface is flat and 

 inclined downward and outward from the suture. The lateral surface is 

 nearly vertical in position and rather strongly concave in outline. The basal 

 surface is much depressed, only slightly convex, and curves into the umbilicus 

 with a somewhat greater inclination. 



The whorls are non-embracing, each subsequent one being attached 

 merely to the basal surface of its predecessor. The suture is rather incon- 

 spicuous and may be either angular or rounded, depending upon its exact 

 position. It usually follows the lower of the two costae which occur upon 

 the lower border of the lateral surface but it may vary from slightly above 

 to slightly below this position. 



