32 Trans. Acad. Sci. of St. Louis. 



arcuate, almost circular in old specimens ; posterior umbonal 

 slope rounded, much produced in old specimens, with rather 

 marked angle at junction with posterior margin; ventral mar- 

 gin disposed to be sulcate ; anterior margin rounded, scarcely 

 produced, not as far forward as the umbonal tips; umbones 

 large, very thick, turgid, somewhat produced beyond the 

 anterior margin, approximating in perfect specimens, minutely 

 undulated at tips; ligament large, thick, long, black, curved 

 parallel to the dorsal margin; lunule large, cordate, black ; 

 cardinal teeth large, heavy, short, bifid in the left and dis- 

 posed to be trifid in the right valve, rough, striate-crenulate, 

 all segments departing at varying angles from a point imme- 

 diately under the apex of the umbones, the dorsal division in 

 the left valve the largest and heaviest, and parallel to the car- 

 dinal teeth ; plate joining cardinal teeth with the laterals short, 

 thick, smooth dorsally, but striate ventrally, margin somewhat 

 crenulate ; lateral teeth long, thick, slightly curved ventrad, 

 striate-crenulate, rough; anterior cicatrices deep, rough- 

 pitted, distinct, outline of the adductor somewhat triangular, 

 that of the protractor pedis elliptical, behind rather than under 

 the adductor ; posterior cicatrices distinct, well and deeply 

 impressed, the adductor concentrically striate ; the retractor 

 pedis circular, pit-like, impressed just below the ends of the 

 lateral teeth; pallial cicatrix broad, crenulate, well impressed 

 throughout, but not deeply impressed anteriorly ; dorsal cica- 

 trices not impressed in the cavity of the beaks, but as a broad 

 row on the posterior margin of plate formed by the cardinal 

 teeth, numerous and rough ; nacre white, in some specimens 

 with occasional brownish blotches, iridescent posteriorly ; 

 dimensions of average mature specimen : length, 95.56 mm. ; 

 breadth, 45.00 mm. ; height, 77.40 mm. 



The only locality in Arkansas which is represented by this 

 form, so far as our collections extend, is the Ouachita river, at 

 Arkadelphia. A single specimen only was secured, but it com- 

 pares well with typical specimens from the Cumberland river. 

 Not far removed from the forms which we believe will properly 

 oroup with this as synonyms are other forms such as Unio 

 plenus Lea and Unio soli dus Lea, the relationships of which 

 are yet somewhat problematical. 



