TERTIARY FOSSILS. 45 



sloping concavely upwards to the suture at a considerably broader 

 angle with the side than in Say's species. Surface bearing several 

 large revolving ribs, one on the angle of the whorl ; on the 

 anterior part of the shell these ribs degenerate by becoming less 

 elevated, but retain their width; between the larger ribs, are a 

 few very faint revolving lines; no similar sculpture exists on the 

 top of the whorls; the whole surface is crossed by pretty distinct 

 strise of growth. Aperture as in decemcostata, except that it is 

 narrower. 



Length, about 2.8 inch ; width, 1.4 inch. 



From the Pliocene of Eagle Prairie, Humboldt County. 



This shell is closely allied to the Eastern decemcostata, and another form recently 

 brought by Mr. Harford from Alaska, the Middendorfii, Cooper = liratus, Mart. 

 Judge Cooper named his species, after a minute comparison of one specimen with 

 130 of the Atlantic decemcostata; subsequently Dr. Carpenter, after a similar in- 

 vestigation, pronounced them identical. I have compared several of the Alaska 

 shells with a fine suite of Say's species in the Philadelphia Academy's collection, 

 and believe I can see a constant difference, though in minute characters only. The 

 present fossil, however, is much longer, more slender, with a higher spire and nar- 

 rower mouth than any of the recent shells. The number of revolving ribs is per- 

 haps about the same as in the living forms, but beyond the first two, they lose their 

 elevation, and the interspace is marked by a smaller rib instead of minute lines. 

 Unfortunately the anterior portion of my single specimen is broken so as to pre- 

 vent as full a comparison of the characters as I could desire. 



K". HUMEROSA, n. S. 

 PI. 14, Fig. 3. 



Shell large, broadly fusiform; spire high; whorls seven or 

 eight convex; body whorl with a rounded shoulder, above which 

 the shell slopes concavely upwards to the suture, which is small 

 and indistinctly marked; no longitudinal ribs, the first two or 

 three whorls show faint nodes, and in some cases the later vo- 

 lutions are wrinkled by irregular growth. Entire surface covered 

 by numerous small, rounded, revolving ribs, with acute inter- 

 spaces, and sometimes exhibiting a tendency to alternation in 



