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 decemcostaia., 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 decemcostaia ; 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 rjbs 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. 



!$". HUMEROSA, D, 8, 

 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 



