THE GASTROPOD GENUS YVANIA 27 



ning of the curvature into the basal surface which occurs at the position of 

 a prominent revolving COSta. Upon its surface is another costa, the distance 

 between the two varying from one third to nearly one half of the width of 

 the lateral slope. Both costae are steeper above than below and the space 

 between them is asymmetrically concave with the deepest portion three quar- 

 ters of the distance to the lower costa. The upper portion of the lateral slope 

 is shallowly concave, due to the prominence of the costa along the outer edge 

 of the slit hand. Growth lines where visible show a strong backward sweep 

 toward the slit band above and a convex curve below, crossing onto the basal 

 surface with a slight backward inclination. 



The basal slope is marked by numerous low step-like spiral costae having 

 steep outer slopes and long inner slopes. The outer of these costae are sep- 

 arated by distances about equal to that which intervenes between the two 

 costae of the lateral surface. There may be two, three, or four such broad 

 interspaces and then they become much narrower and continue so to the col- 

 umellar region. The number of the costae may vary from about ten to fifteen, 

 depending largely upon the spacing of the outer ones. The growth lines are 

 curved, broadly and shallowly concave on the outer half of the basal surface, 

 and continuing nearly straight toward the columella but becoming slightly 

 convex just before they reach it. 



Remarks. — The species Yvania knigliti is closely related to Y. sitprr- 

 crenata from which it may he distinguished by the greater relative width of 

 the band, the fewer revolving costae above it, the lesser prominence of the 

 costa adjacent to the suture, the unornamented character of the band, the 

 more prominent costa upon the outer border of the hand, the concavity of the 

 smooth area just below the hand, the flatter basal surface, the general absence 

 of an umbilicus, the more unequal spacing of the costae of the basal surface, 

 and the inequality in the enlarging of the whorls resulting in a spire having 

 a slightly convex profile. 



The holotype of this species is No. 3 in the collection of the Illinois 

 State Geological Survey. 



Occurrence. — This species occurs abundantly in the I'iasa limestone of 

 St. Louis County, Missouri. The holotype and a series of other specimens 

 were obtained from weathered limestone that was temporarily exposed by 

 grading in the Davis Subdivision, one mile south of Clayton. 



YvANIA SALINENSIS Weller, n. sp. 

 PI. I, nps. 9-10c 



Description. — Shell small, higher than wide, turreted. spiral angle about 

 63°; whorls angular, base slightly convex, umbilicus small or entirely closed; 

 last whorl forms about two fifths the heighl of the shell: surface marked 



