WHITE, ] LARAMIE FOSSILS. 63 
postero-dorsal portion of the shell behind the umbonal ridges compressed, 
and in young examples it is there subalate; front portion of the shell 
moderately gibbous, and between the front and the umbonal ridges the 
sides are flattened; anterior margin regularly but somewhat narrowly 
rounded down to the basal margin, which latter margin is straightened 
along its middle portion; postero-basal margin somewhat narrowly 
rounded, and extended upward and backward to the postero-dorsal mar- 
gin; the latter margin sometimes truncated obliquely downward and 
backward and sometimes so rounded as to give an approximately square 
truncation to the posterior end of the shell; hinge line long and straight. 
Surface marked only by the ordinary lines of growth except all that 
portion which lies behind and above the umbonal ridges. This portion 
of the surface is marked by numerous sharply raised irregular lines or 
narrow ridges, with the intervening spaces wider than the ridges thein- 
selves, which, beginning almost imperceptibly just behind the umbonal 
ridge, extend backward with a greater or less upward curve to the dor- 
sal and posterior borders. They usually constitute a conspicuous surface 
feature of the shell, but in the case of old examples they appear to have 
become obsolete. Their character is similar to that of markings upon 
corresponding parts of U. senectus, and U. primevus, especially the latter. 
Length of the largest discovered example, 82 millimeters; height of 
the same at mid-length, 48 millimeters; thickness, about 32 millimeters. 
The specific name is given in honor of Mr. Charles Aldrich, formerly a 
member of this survey. 
Position and locality—Upper portion of the Laramie Group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where it is associated 
with a majority of the following-described species of Unio, and also with 
several other molluscan species. 
UNIO GONIAMBONATUS White. 
Plate 29, figs. 1 a and b. 
Unio goniambonatus White, 1878, Bull. U. 8. Geol. Sur. Terr., vol. iv, p. 709. 
Shell of medium size, transversely elongate, subtrihedral in marginal 
outline, being rapidly narrowed posteriorly from the anterior portion ; 
valves moderately gibbous, their greatest convexity being in front of 
the mid-length and above the mid-height; test moderately thick; um- 
bones prominent, but not much raised above the cardinal margin; 
beaks situated near the anterior end, depressed, a little flattened; um- 
bonal ridge distinct, angular, and so prominent as to produce a flattened 
or even slightly concave space between it and the cardinal margin, giv- 
ing the whole back of the shell a broadly flattened aspect; front margin 
regularly rounded from beneath the beaks to the basal margin, which 
latter margin is nearly straight in some examples, but in others broadly 
convex; postero-basal margin narrowly, and sometimes irregularly, 
rounded to the postero-dorsal margin; the latter margin forming a 
gentle slope downward and backward from the posterior end of the 
hinge; cardinal margin nearly straight, and occupying about two-thirds 
the whole length of the shell. 
Surface marked only by the ordinary lines and imbrications of growth, 
but usually the angular umbonal ridges are each cut across by three or 
four short, distinct folds or ridges, and their intervening furrows, extend- 
ing obliquely inward and backward, being scarcely perceptible in front 
of the umbonal ridge, and becoming obsolete before reaching the pos- 
tero-dorsal margin, or at least producing only slight undulations in it. 
Length, 58 millimeters; height from base to umbones, 34 millimeters; 
thickness, both valves together, 28 millimeters. 
