Call — The Unionidce of Arkansas. 39 



coquille a nacre pourpree avec des laches irregulieres d'un 

 vert violatre, sur-tout sous les crochets. Largeur de mon ex- 

 emplaire, 139 millimetres. La dent cardinale est epaisse mais 

 de taille mediocre. L' autre dent est tres finement crenelee." 



The reference of this form to the rivers of Africa is, of 

 course, entirely incorrect ; its general resemblance to Pleiodon 

 may have caused the statement. 



Lea's Unio ateris the female of this species, while his Unio 

 coloradoensis is based upon a large male. Conrad's Unio 

 poitlsonii is also a male. Again Say described, for his form, 

 the female. 



The shell may be described as follows from the appearances 

 presented by mature and large specimens taken in the Little 

 Arkansas river, Kansas, by Mr. J. R. Mead. 



Shell large, heavy, smooth, striate, posteriorly elliptical, 

 convex, thick, very thick anteriorly, rounded before, obtusely 

 biangulate behind ; epidermis thick, black, striate towards the 

 margin, and on the posterio-dorsal slope, umbonal slope shin- 

 ing, eradiate or obscurely rayed, the rays seen only on the 

 thin margins of the old specimens, in the young as many cap- 

 illary lines over the whole disk of the umbones ; lines of growth 

 conspicuous, numerous and crowded, in old specimens form- 

 ing slightly raised ridges, which are parallel to the margins but 

 which, posteriorly, form imbrications, these are often broken 

 and give the shell an exceedingly rough appearance; dorso- 

 posterior margin slightly curved ; posterior, umbonal slope 

 marked by two carinas, one of which is very prominent, the 

 other but slightly indicated, these, at the posterior margin, in- 

 dicate the positions of the incurrent and excurrent orifices and 

 render the margin biangulate; umbones large, prominent, 

 rounded, always so eroded in the old shell as not to disclose the 

 character of the undulations, but nearly perfect, young speci- 

 mens present indications of apiculate folds ; ligament long, 

 thick, black, scarious, rough ; cardinal teeth single in the right 

 and double in the left valve, of young specimens, but disposed 

 to be trifid in the right valve, in large and old specimens, erect, 

 dentate, rough, triangular, the anterior portion, in the left 

 valve, the larger; lateral teeth long, straight, thick-lamellar, 

 smooth, but often slightly crenulate on the margin, separated 



