Call — The Unionidce of Arkansas. 35 



river, at Benton. It no doubt occurs in abundance in many 

 other streams in the State. It is usually a very abundant 

 shell, preferring the muddy banks of bayous and sluggishly 

 flowing portions of streams, where it may be found most 

 commonly buried in the mud. There is but the minute open- 

 ing leading from the siphons to indicate its presence. 



So few animals of the Unionidce have been described that it 

 may not be amiss to insert at this place a description of the 

 animal of Unio parvus based upon the examination of speci- 

 mens taken in the Des Moines river, in central Iowa. 



Animal of Unio parvus. Color of the mass whitish ; tenta- 

 cular portion of mantle dark brown, ending in a caruncle; 

 labial palps large, white, triangular, united at base and 

 partially so over the posterior margin ; external ctenidium 

 smaller than the internal, thicker and larger at the posterior ex- 

 tremity, which is rounded, and on the margin, which is marked 

 by a double row of minute, white papilla?; ctenidia united 

 above throughout their entire length; free below; internal 

 ctenidium white, ovate. 



The mass of the animal within the cavity of the beaks is 

 light brown owing to the color of the large liver which shows 

 through the thin tissues separating it from the cavity of the 

 branchiae. 



The chief anatomical peculiarity is the presence of the 

 caruncle in the female; this is somewhat separated from 

 the main tentacular mass and is supported by a slender 

 pedicel. 



Unio phaseolus Hildreth. 



Am. Jour. Sci. and Arts, 1st series, Vol. XIV, 1828, p. 

 283, Fig. 14; Say, in American Conchology, 1830, Plate 

 22; Reeve, in Conchologia Iconica Unio Plate LXXIII, 

 Fig. 378, 1868 ; Lea figures splendidly the soft parts in 

 Jour. Acad. Nat. Sci. Phila., 2d series, Vol. IV, Plate 

 XXIX, Fig. 101. 



Unio planulatus Lea. Trans. Am. Philos. Soc, Vol. 

 Ill, 1829, p. 431, PI. IX, Fig. 13. 



Unio camelus Lea. Trans. Am. Philos. Soc, Vol. V, 

 1834, p. 102, PI. XV, Fig. 45. Eeeve, in Conchologia 



