and shining, reflecting the face of the observer ; young 

 shells are splendent, having a much stronger lustre on the 

 outside than on the inside ; cardinal teeth broad, promi- 

 nent and obliquely flattened ; lateral teeth broad, elevated 

 and terminating abruptly before ; cicatrices large ; cavity 

 of the beaks unusually large ; naker pearly white j sur- 

 face smooth, but not highly polished." 



OBSERVATIONS. 



I quote the above description from Barnes' very useful 

 essay "On the genera Unio and Alasmodonta," in which 

 this 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 intermedi- 

 ate 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 colour 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 anterior side, some specimens are 

 beautifully tinted with a pink colour, but this is not very 

 common ; they are generally white. It is very common 

 in our western waters. The U. occidens of Lea may per- 

 haps be U. ventricosus, Var. b. Barnes. 



REFERENCE TO THE PLATE. 



The plate exhibits an exterior and interior view of the ehcll. 

 PI. 32. 



