12 THE GASTROPOD GENUS YVANIA 



and the similar type of ornamentation, leaves little doubt as to the reference 

 of these forms to Yvania. 



The two species in this group are not as closely similar as those in the 

 other three groups. Nevertheless they possess certain conspicuous common 

 characters, such as the more concave and unornamented lateral surface of 

 the whorl, and the smooth central portion of the basal surface of the whorl 

 occurring between the outer and inner series of spiral costae. 



4. Group of Yvania subconstricla. This group which also contains 

 Y. (jiff or di is sharply distinguished from all of the preceding. In fact the 

 other groups are probably much more closely related to each other than this 

 one is to any of them. The concave form of the upper sloping surface of 

 the whorl is reminiscent of Y . gurleyi and its relatives, but in this group, the 

 slit band is flat and continues uniformly in the slope of the concave portion of 

 the upper surface of the whorl, whereas in the group of Y . gurleyi it is 

 concave and is inclined downward from this slope. 



The group of Y. subconstri'cta differs from the others in two important 

 particulars. Growth lines, represented by fine transverse costae, which are 

 prominent on well preserved specimens in the spaces between the revolving 

 ribs, are more conspicuously developed. The transverse costae have not been 

 seen to cross the revolving costae, but the latter are rendered somewhat irreg- 

 ular and lack the smoothness characteristic of the other groups. This is most 

 conspicuous upon the costa at the angle adjacent to the suture, and in some 

 specimens on the next lower one, which are rendered somewhat nodose. A 

 less noticeable character, but probably more significant, is the linguliform 

 outline of the aperture on the lateral slope which is produced by a compara- 

 tively sharp concave curvature below. The basal surface of the whorl is 

 also considerably more convex than in any of the preceding groups. 



Species Tentatively Referred to YVANIA 



A search through the literature has failed to reveal a single species from 

 the American Carboniferous, other than those described in this paper, which 

 may with certainty be included in the genus Yvania. There are, however, 

 four species which should be provisionally placed in this genus on account 

 of certain similarities, but the descriptions and illustrations of them are 

 inadequate for a precise determination of their relationships. Of these four 

 species, one, Worthcnia alamillana Girty, is described from the Upper Penn- 

 svlvanian of New Mexico, and the other three, Phurotomaria arcnarea Girty, 

 P. dclazvarcnsis Girty, and P. agnostica Beede, are from the Permian of 

 Texas and Oklahoma. No species even suggestive of this genus has been 

 described from the Mississippian. Apparently Yvania did not appear in 

 America until Pennsylvania!! time. 



