WHITE. ] JURASSIC FOSSILS. 153 
intact, and appearing as if it had never borne a callus. I am now con- 
vinced, however, from the character of the shells and also of the im- 
bedding matrix, that they never had any such thickening of the inner 
lip as characterizes the Neritide. 
LYOSOMA POWELLI White. 
Plate 38, figs. 6 a, b, c, and d. 
Neritina?? powelli White, 1876, Powell’s Rep. Geol. Uinta Mts., p. 110. 
Shell obliquely subovate in outline by apertural view; volutions about 
three or three and a half, rapidly increasing in size, the last one much 
expanded; spire depressed, the apex scarcely appearing by side view 
of the shell; suture slightly impressed; aperture large, broadly subeir- 
cular or obscurely subtetrahedral; a shallow revolving depression marks 
the median portion of the upper side of the outer volution, and between 
it and the suture there is a corresponding revolving convexity. Another 
revolving prominence outside of the depression gives the volution a 
good degree of obtuse angularity there. 
Surface marked by ordinary lines of growth and also by somewhat 
prominent folds parallel with them, the folds being stronger upon the 
revolving prominences than elsewhere and especially upon the outer one, 
below which they are obsolete or absent. 
Greatest diameter of the largest example, 28 millimeters; breadth of 
the same, 20 millimeters; height, the aperture resting upon the table, 
15 millimeters. 
Position and locality.—Jurassic strata, mouth of Thistle Creek, Span- 
ish Fork Caton, Utah, where it was collected by Prof. J. W. Powell. 
