38 THE GASTROPOD GENUS YVANIA 



are approximately vertical or slightly convex forward but they swing strongly 

 backward on the remainder of the lateral slope and meet the edge of the slit 

 band at an acute angle. The growth lines are conspicuous only in the inter- 

 spaces and are not visible crossing the revolving costae except for slight 

 irregularities which they produce. 



The basal surface carries about ten to twelve rounded spiral costae 

 separated by concave interspaces. In rare cases this number may be as 

 great at seventeen or eighteen. The interspaces are commonly slightly wider 

 in the central portion of the basal surface of the whorl. The outermost 

 costa generally occurs very close to the one which marks the angle between 

 the lateral and basal surfaces. On those specimens where an unusually large 

 number of costae are present upon the basal surface they are much more 

 closely spaced in the columellar region than over the remainder of the base. 

 Lines of growth are fairly distinct and appear as fine transverse costae in 

 the interspaces. From the outer angle they sweep very gently backward in 

 a broad shallow concave curve to the middle part of the basal surface, and 

 become slightly convex in the vicinity of the columella. 



Remarks. — The species Yvania giffordi is very closely related to Y. 

 sitbconstricla which occurs principally at somewhat lower horizons. Y. 

 giffordi may be distinguished principally by its more acute spire but also by 

 its relatively narrower slit band and the horizontal area adjacent to the suture. 

 At most localities the specimens are badly exfoliated and the surface features 

 cannot be observed. 



The holotype of this species is No. 2535 in the paleontological collection 

 of the Illinois State Museum. 



Occurrence. — This species was originally described from Peoria County, 

 Illinois, without notation as to either locality or horizon. This gastropod 

 is one of the most characteristic members of the Lonsdale limestone fauna 

 and is present in most o<f the collections from this horizon in Peoria County 

 and vicinity. Two specimens in the Gurley Collection preserved in Walker 

 Museum at the University of Chicago are labeled as having been obtained 

 from above coal No. 7 in Peoria County. The accuracy of this reference 

 seems doubtful as the specimens are preserved much like some of those 

 obtained from the Lonsdale limestone and this latter horizon is the only one 

 from which I have obtained this species. 



