CqU — The UnionidcB of Arkansas. 27 



beaks of the young, and this statement is true as well of old 

 and perfect specimens, are beautifully marked with ridges 

 that are concentric, or better, perhaps, angulated, with the very 

 obtuse angle pointing toward the tip of the umbones. In 

 Unto ligamentiniis this character is not so marked and differs 

 in the degree of fineness of these ridges. 



The following description is based upon a series of shells 

 taken from the Des Moines river, in central Iowa : — 



Shell large, elongate, somewhat inflated, rather thin, circu- 

 larly rounded before, elliptically rounded behind, the male 

 often somewhat pointed posteriorly, female more tumid poste- 

 riorly, emarginate ventrally; epidermis light horn color, 

 polished and shining, usually abundantly rayed with narrow, 

 bright, green, crenulate, somewhat curved rays which depart 

 from the beaks and cover more or less closely the posterior 

 three-fourths of the disk, these are often wanting, especially 

 in old specimens: lunule long, narrow ; ligament long, thin, 

 light horn color; hinge margin nearly straight or very slightly 

 arcuate ; umbones prominent, approximate, concentrically 

 wrinkled, the wrinkles being angulate and the apices of each 

 pointing toward apex of the umbone, light, nearly white, in 

 color when the epidermis is perfect ; cardinal teeth double in 

 both valves, equal only in the left valve, rather small, thin, 

 acutely serrate, all directed anteriorly ; lateral teeth long, 

 thin, lamellar, striate, nearly or quite straight ; dorsal plate 

 smooth, short, rounded, thin ; anterior cicatrices distinct, that 

 of the adductor rather deep, large, striate, irregularly im- 

 pressed, that of the protractor pedis well impressed but not 

 deep ; posterior cicatrices confluent, very slightly impressed, 

 smooth, iridescent; pallial cicatrix well impressed before, 

 broad and shallow, or scarcely impressed, behind; dorsal cica- 

 trices numerous, impressed as deep pits in an irregular row in 

 the center of the cavity of the beaks ; cavity of the beaks 

 rather shallow; nacre pure white, sometimes somewhat 

 iridescent posteriorly. 



The swollen outline of the full grown female apparently led 

 to the description of this form under the name of Unto sili- 

 quoideus by Dr. Barnes. This variety in the Mississippi and 

 its larger tributaries, especially those which enter it from the 



