20 



ligation may add one or two species to our Unionidoe, 

 or it may reduce them to two Unios, and one Anodonta. 



This group of bivalve shells represent our largest 

 species of fresh-water raollusoa. lu this country it is 

 but a small family with respect to different species, yet 

 in America it constitutes a very varied one, and in the 

 United States alone there are two hundred species. 



The Unionidoe are all equivalve. They are covered 

 with a brilliant epidermis, which gives them a very 

 shining appearance. The interior of their valves have 

 a pearl-like surface. All of them are inhabitants of 

 fresh water, crawling on the bottom, partly buried in 

 the mud. The shells of the females are more ventri- 

 cose than those of the males. 



Unio. 



Very variable in form ; frequently of great strength ; 

 the ligament external, and having primary teeth on the 

 hinge. 



Unio tlmidus (The Tumid Union). Retzius. 

 Figure 9. 



The valves of this Unio are somewhat of an oblong- 

 ovate shape, being unequally ventricose, and the um- 

 bones swollen and wrinkled. The colour of the shin- 



