THE NATURAL HISTORY SURVEY. 47 
FAMILY UNIONID-. 
The author is greatly indebted to Mr. Charles T. Simpson, of the United 
States National Museum, Washington, D. C., who has revised the manuscript 
of this family and has made many valuable suggestions regarding the classifi- 
cation and anatomy of the species and genera of the region covered by this 
work. In some groups Mr. Simpson has not fully worked out the anatomy 
andsynonymy. In those cases where Mr. Simpson has described a genus or 
section, or has added notes regarding the anatomy, his name has been placed 
in parenthesis after such description or note. 
«Shell usually equivalve and inequilateral, smooth or va- 
riously sculptured, angular or rounded, symphynote or nonsym- 
phynote, covered with a thick epidermis, which may be green, 
brown, yellowish, black, rayed, or variously painted; deaks usually 
sculptured with concentric ridges, corrugations, chevron shaped 
or radiacal patterns, or pustules, offen showing remains of the 
nuclear shell; ligament opisthodetic, well developed, external 
except when the shell is symphynote. Interior nacreous; with 
or without hinge teeth, dut showing vestiges of them in every genus; 
when present always schizodont and arranged as cardinals, laterals 
(pseudocardinals and pseudolaterals), or doth; adductor scars 
generally distinct, the anterior commonly impressed; pallial line 
simple and generally well marked; prismatic border usually nar- 
row and not conspicuous. 
“Animal: Ladial palpi almost always wider than long, having 
the upper parts of the posterior margins united; anal opening usually 
separated from the superanal. Mantle either free or closed pos- 
teriorly to form a branchial opening. Zmbdryo a glochidium, the 
soft parts being inclosed in a pouch shaped bivalve shell, either 
with or without hooks, and borne in the inner or outer, or in all 
four leaves of the branchiz, which are modified to form a mar- 
supium.”’* 
Of the 1,000 recent species of this family, over one-half 
inhabit the rivers and lakes of the United States. Like most 
fresh-water shells, the umbones of this group are nearly always 
eroded by the carbonic acid gas (CO,) which is dissolved in the 
water. The Anodontas are generally found in still bodies of 
water, on muddy bottoms, while the Unios prefer, as a rule, the 
bed of running streams. The Mississippi Valley is the metrop- 
olis of this interesting family of mollusks, and it is here that 
they are found in their greatest development, beauty and variety. 
The present area, with its numerous lakes and streams, affords 
an excellent locality for this group and the species found are 
* Simpson, Classification of Pearly Fresh-water Mussels, p. 318. 
