36 THE GASTROPOD GENUS YVANIA 



Yvania giffordi (Worthen) 

 PI. II, figs. 6-8b; PI. Ill, figs. 5, 11 



1884 Pleurotomaria giffordi. Worthen, Illinois State Mus. Nat. Hist., Bull. 1 

 p. 5. 



Coal Measures : Peoria, Illinois. 



1890 Pleurotomaria giffordi. Worthen, Geol. Survey of Illinois, vol. 8, p. 135. 

 PI. 23, figs. 8-8a. 



Coal Measures : Peoria County, Illinois. 



Description. — Shell small to medium sized, one and one half times as 

 high as wide, spiral angle about 45 c to 50°; whorls angulated, base convex, 

 not umbilicated ; last whorl forms slightly less than half the height of the 

 shell ; surface marked by numerous revolving costae which are nodose adja- 

 cent to the suture, and by transverse lines of growth; slit band prominent, 

 flat, located on upper sloping surface of whorl and adjacent to the angle. 



The dimensions of an average individual are : height of entire shell 

 9.0 mm. ; height of last whorl 4.0 mm. ; width at base 6.0 mm. It is composed 

 of about seven whorls whose development is probably very similar to that 

 of Y. subconstricta. The specimens examined, however, are not sufficiently 

 well preserved to show these details. 



The mature whorls are subrhomboidal in outline and of about equal 

 height and width. The outer surface is divided into three parts by two angu- 

 lations in addition to which a third minor angulation occurs adjacent to the 

 suture along the upper border of the whorl. The upper of the two main 

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

 angle of about 140°, occurring below the suture by slightly more than one 

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

 first by somewhat less than one third the height of the whorl. It is not as 

 sharp as the upper one but marks the beginning of the sharp inward curve 

 of the basal surface. The upper slope is constricted and concave just above 

 the slit band. From this constriction the surface slopes outward to the 

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

 to the suture. This last angulation is one of about 115° and is separated 

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

 whorl is concave and is approximately vertical in position. 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 its 

 predecessor. The suture is sharp, narrowly concave to angular, and may be 

 slightly excavated. In the upper whorls of the spire it rather closely follows 

 the position of the costa that marks the lower limit of the lateral surface of 

 the preceding whorl, but in the later turns it commonly follows a somewhat 



