1887.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 499 



Common in Currant river, Shannon county, Mo., and in Jack's fork 

 and Big creek, tributaries to it. 



In general form this shell resembles Unio lenticularis, Lea, and Unio 

 connasaugemis, Lea, from Tennessee, with which species it groups. 

 Owing to the degeneration of the alcohol in which a number of speci- 

 mens were collected it is impossible to give any account of the soft 

 parts, which were destroyed. My note-book, however, shows that the 

 ova are pinkish, and that the general characters of the ctenidia are 

 similar to those exhibited by Unio rubiginosus, Lea, which species this 

 shell in no other particulars at all resembles. 



A single individual among the hundred or more found exhibited the 

 cardinal teeth normally disposed, but the lateral teeth were reversed ; 

 i. e., single in the left and double in the right valve. Instances of a 

 similar partial reversion are not uncommon, while complete reversion, 

 though rare, is exemplified in a number of common species. 



Unio breviculus, sp. nov. (Plate xxviii, Figs 1, la, Ib male ; 2, 2a, 2b female). 



Shell smooth, ovate elliptical, inequilateral, subiuflated, biangular 

 posteriorly, circularly rounded before, somewhat incrassate ; umbones 

 slightly elevated, so much eroded that minute characters are inde- 

 terminate ; ligametit large, thick, black, or dark brown ; epidermis yel- 

 lowish horn-color, smooth, polished, rayed with dark green over the 

 whole disk, the rays often interrupted by the lines of growth, which are 

 numerous, but somewhat indistinct; umbonal slope rounded, depressed 

 in the male, slightly elevated in the female 5 posterior outline emargin- 

 ate in the female ventral of the siphonal area, dorsal outline rounded ; 

 cardinal teeth double in the left and single in the right valve, short, 

 erect, triangular, solid, smooth, or scarcely crenulate ; plate connecting 

 laterals with cardinal teeth thick, somewhat arched ; lateral teeth 

 rather short, thick, slightly curved, smooth ; anterior cicatrices dis- 

 tinct, large, deeply impressed ; posterior cicatrices confluent, well im- 

 pressed, that of the retractor pedis muscle at tip of base of lateral tooth 

 but not on it ; dorsal cicatrices numerous and deeply impressed in the 

 cavity of the umbones ; nacre salmon colored, occasionally white. 

 Length 71.00 mm ; breadth 27. 20 mm ; height 45.50 min . 



Animal dirty yellowish white ; labial palps short, ovately triangular, 

 adherent at base, laterally united so as to form an oval groove, midway 

 from the extremities of which is placed the mouth. In the specimens 

 examined only the anterior one- third of the external branchiae contained 

 ova. This portion was characterized by the heavy deposit of pigment- 

 ary matter at the apex of the chambers, while the remaining margins of 

 the branchia) were uniform in coloration with the mass of the animal. 

 The posterior borders of the mantle were, as usual, differentiated into 

 a series of tentacular folds ; those surrounding the incurrent and ex- 

 current orifices were yellow and brown the remainder were black. 



There is no well-known Unio with which this form is comparable. 



