189 
species was first distinguislied from the U. ovatus, nob., to which 
it is very closely allied, and appears to be absolutely connected by 
a gradual transition of intermediate varieties. The most striking 
difference seems to be the more depressed anterior slope of the 
ovatus. Some old specimens and varieties are transversely more 
elongated, as is represented in Barnes’ figure c. The color also is 
more or less yellow, radiated with green, or tinged with reddish 
brown, particularly on the umbo, and sometimes extending over 
the whole surface, almost obliterating the rays. On the inner an- 
terior side, some specimens are beautifully tinted with a pink color, 
but this is not very common j they are generally white. It is very 
common in our western waters. The U. occidens of Lea may 
perhaps be U. ventricosus, var. b, Barnes. PI. 32. 
Unio interruptus. — Specific character. An elevated um- 
bonial slope, separating folds of the disk from folds of the anterior 
margin. 
Unio interruptus, nob. Transylvania Journal, vol. ^,p. 525. 
Desc. Shell transversely oblong oval or subrhomboidal, black- 
ish brown ; an elevated rib extends from the apex to the anterior 
basal angle along the umbonial slope; anterior half of the disk 
with somewhat oblique undulations extending nearly to the um- 
bonial rib ; anterior to the umbonial rib are several arquated un- 
dulations, which do not reach the edge of the shell, and are cut 
by an impressed line which is nearly parallel to the rib ; umbo 
compressed; beaks not elevated, deeply eroded; posterior margin 
very short, rounded; ligament margin rectilinear, the anterior 
angle elevated, obtuse ; anterior slope very oblique ; base recti- 
linear, parallel to the ligament margin ; anterior basal angle some- 
what rostrated ; cavity of the hinge membranes (or excavated 
lunule) fusiform; within purplish, iridescent; cardinal teeth direct, 
deeply granulated and lineated over their whole surface ; lateral 
teeth entire ; posterior cicatrices rough, the smaller one orbicular ; 
anterior edge blackish. 
Obs. The species to which this makes the nearest approach, is 
undoubtedly the undidatus, Barnes ; but it is more transverse, more 
oblique anteriorly ; the edges of the hinge margin and of the an- 
terior slope are undulated, the umbonial slope is elevated into a 
distinct separating rib ; and it is of a fine livid purple color within. 
The eroded surface of the umbones is perlaceous, iridescent, and 
at the apices it exhibits a dull waxen yellow surface. 
