The Shell-Collector’s Handbook. pe) 
CHAPTER II. 
THe ANATOMY OF A FRESH-WATER MUSSEL. 
THe Swevtt.—The shell of a Lamellibranch—for 
that is the class to which the Fresh-water Mussel 
belongs—is a bivalve. The valves are united to one 
another by an elastic /égament, and near to this liga- 
ment is a rounded blunt prominence of the shell 
known as the wmbo. The external surface of the 
Shell is marked by lines—lines of growth—running 
concentric to the umbo. The interior of each valve is 
white and iridescent, and near to its dorsal border 
are two depressions which mark the insertion 
of the anterior and posterior adductor muscles, 
called respectively the anterior and posterior adductor 
impressions. Joining these two impressions a faint 
line can be observed—the pallial line—indicating the 
attachment of the pallial muscle of the mantle. Con- 
tinuous with the posterior border of the anterior 
adductor impression is another depression—the 
anterior retractor impression—and near to it a 
distinct fourth impression will be noticed—the 
protractor «impression. The posterior retractor im- 
pression is continuous with the anterior portion of the 
posterior adductor impression. At the dorsal margin 
the valves are united together along a line, which is 
