THE GASTROPOD GENUS YVANIA 11 



Relations of YVANIA 



The genus Yvania is apparently most nearly related to Gosseletina Bayle 

 [Gossclctia de Koninck) as represented by the American Coal Measures 

 species Gosseletina spironcma ( Meek and Worthen)-. The species of Yvania 

 described in the following pages, whose embryonic development is known, 

 pass through a stage which is essentially similar to the mature form of 

 Gosseletina in that the slit hand is situated well up above the middle of the 

 rounded whorl, and that the lines of growth above the hand are more or less 

 linguliforni and meet the suture at an angle which is acute in a backward 

 direction. These two groups are probably descended from the same ancestral 

 stock. Of them Gosseletina is the more primitive as it lacks the angulation 

 which is developed in the later whorls in Yvania. 



Grouping of American Species 



Nine species of Yvania are descrihed in the following pages. Four of 

 these were descrihed and illustrated by Meek and Worthen in the first series 

 of reports of the Illinois Geological Survey, the remaining five are new. 

 The species of Meek and Worthen have been practically unknown to paleon- 

 tologists up to this time; the original descriptions and illustrations are rather 

 inadequate and no later notice of them appears in paleontologic literature. 



The nine species of Yvania descrihed in this paper may he subdivided 

 into four groups as follows: 



1. Group of Yvania gurleyi. These forms are characterized by a some- 

 what turreted spire which is ornamented by numerous simple and smooth 

 revolving costae. The upper sloping surface of the whorl is gently concave 

 between the slit hand and a costa which lies upon an angle adjacent to the 

 suture. The form of the aj>erture beneath the sinus of the hand is broadly 

 and smoothly curved. The group also includes )*. inclinata and Y. pitsilla. 



2. Group of Yvania supercrenata. This group differs from the pre- 

 ceding principally in that the upper sloping surface of the whorl is flat rather 

 than concave. These two groups are not as closely related as might appear 

 from superficial examination as the embryonic development is somewhat 

 different. Y. knighti is also included in this group. 



3. Group of Yvania adamsi. This group also contains Y. salincnsis. 

 These species are confidently referred to Yvania even though growth lines 

 have not been observed on any of the specimens examined and therefore it is 

 not certainly known that the aperture above the slit band has the shape char- 

 acteristic of this genus. However exfoliated specimens clearly show that the 

 slit band is located upon the upper flattened surface of the whorl and adjacent 

 to the periphery. This character, together with the similar form of the shells 



