Descriptions of Xeiv Species and Xeiv Varieties 225 



finer than in disjunctum. Aperture rounded above and below, slightly ex- 

 panded where it joins the body whorl. A distinct callus on the parietal wall, 

 often making the aperture and lip continuous. A slight thickening inside 

 outer lip, bordered by brown band. 



Type Locality. Yellowstone Lake, Wyoming. 



Type Material. Five specimens, collected by Dr. Curtis, deposited in 

 the United States National Museum, Accession no. 30207a. Holotype here 

 shown as fig. 13 on plate 96; paratypes as figs. 14-17 on same plate. 



H. s. per disjunctum is similar to disjunctum but is much smaller, about 

 the size of oregonense, but lacks the characteristic shape of the aperture of 

 the last named form. The dorsal carina and the more uniformly and evenly 

 coiled whorls, without rapid increase in size distinguish it from oregonense. 

 Appears at first like immature disjunctum but is uniformly smaller; never 

 has the axial height of disjunctum. ]\Iany specimens have lost the epi- 

 dermis, exposing white, chalky shell. 



HelisoTTia trivolvis marshalli, New Variety 



Plate 87, fig.s. 11-15 



Shell compressed, whorls four and one-half, rapidly increasing in diameter, 

 the body whorl notably expanding toward the aperture. Sutures well 

 marked above and below. Sculpture of coarse lines of growth, thread-like; 

 spiral lines very indistinct or wanting. Spire depression small, narrow, 

 shallow (less than one-half the width of whorl) , its border rounded, with- 

 out carina in adult specimens, carina slightly developed in young and 

 immature specimens. Spire flat in young individuals. Ventral surface flat- 

 tened, the whorls coiled in one plane in mature specimens, three whorls 

 visible, the umbilical whorls with a more or less acute carina. Sutures deep. 

 In young and immature individuals the carina is sharp and the basal whorl 

 about the umbilicus is raised to form an inverted V; whorls rounded ven- 

 trally in half grown and immature specimens. Aperture wider than high, 

 long oval, the dorsal margin forming an inverted V near junction with body 

 whorls; the ventral margin flat, straight, extending well beyond the position 

 of the dorsal margin on the body whorl. Outer lip sharp, with slight thick- 

 ening within, margined by reddish line. Surface of shell light or dark horn. 



Helisoma trivolvis marshalli differs from jallax in that the new variety 

 has the body whorls more expanded toward the aperture; the dorsal de- 

 pression is smaller; color is light or dark horn, not greenish; the aperture 

 is wider and more ovate. 



