THE NAUTILUS. 119 



horn colored, frequently stained reddish from the iron oxide in the 

 water; surface shining, lines of growth very heavy, more or less rib- 

 like, equidistant ; spiral lines absent or very faint ; whorls 4^, closely 

 coiled ; spire flat, or a little convex, all but the last whorl coiled in 

 the same plane ; umbilicus wide and deep, somewhat funnel-shaped, 

 exhibiting two and one-half whorls; the whorls are sharply carinated 

 above and below, the last whorl being particularly so marked ; this 

 carination of the whorls causes a flattening of the periphery ; the last 

 whorl at a point about midway suddenly bends upward, causing the 

 aperture to be elevated half the diameter of the whorl above the spire, 

 and also causing the umbilicus to form a crater-like contour when 

 viewed laterally ; aperture campanulate, wider below and angled 

 above ; the sutures are distinctly marked, even channeled in some 

 specimens; outer lip sharp; inner lip appressed to body whorl, 

 which is covered with a callus. 



This was at first thought to be Ball's rudentis, but by a compar- 

 ison with his description' and with specimens believed to be au- 

 thentic, it was seen to be quite different. The spire of nidentis is 

 flat and elevated above the last whorl, forming, as observed by Ball, 

 an aspect like a miniature coiled hawser. In smithii the spire is 

 depressed and more or less funnel-shaped. In nidentis the last whorl 

 is deflected, being on a plane or a trifle below the base of the shell, 

 while in smithii the last whorl is elevated far above the plane of the 

 spire. The umbilical region is also very different in smithii. The 

 sharply carinated whorls also afford a striking difference. Typical 

 campanulutus is smaller, the whorls are usually coiled in the same 

 plane and the whorls are rounded and not sharply angulated. P. 

 smithii was at first thought to be a good species, but the presence of 

 the typical form in the lake, which shows marked variation toward 

 the smithii type, leads to its restriction as a strongly marked variety. 



> Alaska Moll., p. 90. 



