189 



species was first distinguished 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 

 diiFerence 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 ; they are generally white. It is very 

 common in our western waters. The U. occidens of Lea may 

 perhaps be U. ventricoms, var. h, 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. 4, 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. 



Ohs. The species to which this makes the nearest approach, is 

 undoubtedly the undulatus, 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 u 

 distinct separating rib ; and it is of a fine livid purple color within. 

 The eroded surface of the umbones is pcrlaceous, iridescent, and 

 at the apices it exhibits a dull waxen yellow surface. 



