TERTIARY PERIOD. 213 



The longitudinal plications vary somewhat in character and distinct- 

 ness in different shells and also in different parts of the same shell ; they 

 are smaller and more numerous upon the small volutions, and become large, 

 distant, and prominent on the larger ones ; all are curved a little outward 

 and forward, none having the exact direction of the axis. 



The surface of the shell is marked by numerous small, raised, revolv- 

 ing lines, which are more prominent, and sometimes even slightly spinous, 

 where they cross the longitudinal plications. 



Length of the largest specimen in the collection, thirty-two millime- 

 ters ; breadth of the last volution, nine millimeters. 



This shell closely resembles the figure given by Hall of G. tenera 

 {■zzCerithium tenenim Hall, Expl. Exp. Oregon and Northern California, 308), 

 and is, not without some hesitation, referred to that species. There is, 

 however, great variation among the related forms of this genus, as found in 

 the Tertiary rocks of that part of the country, and satisfactory specific dis- 

 crimination is often impracticable. 



Position and locality. — Strata probably of Eocene Tertiary age ; head 

 of Soldier's Fork, Utah. 



*Goniobasis Nebrascensis Meek and Hayden. 

 Plate XXI, fig. 9 a, h, and c. 

 Goniobasis Nebrascensis Meek and Hayden, ]856, Proc. Acad. Nat. Sci. Phila., 124. 



Shell pointed-subovate, elongate ; spire moderately elevated, conical ; 

 volutions five to seven, depressed-convex ; suture distinct, impressed ; aper- 

 ture subovate, narrow behind, abruptly rounded in front ; outer lip sinuous, 

 somewhat prominent toward the front ; columella curved. 



Surface marked by the usual fine lines of growth, and also by a number 

 of more or less distinctly raised revolving lines, which are more distinct 

 upon the middle of the volutions than elsewhere ; one line about the middle 

 of the volution is usually a little more prominent than any of the others, 

 which gives it an angulated or subcarinate appearance. 



Length of the largest example in the collection, seventeen millimeters ; 



*Tbe general form of this shell differs somewhat from the typical forms of Goniobasis, and approaches 

 somewhat that of Lioplacodes ; hut, so far as I have beeu able to observe, the aperture is much more 

 nearly like that of the former genus thau the latter. 



