384 TESTACEOUS MOLLUSCA. 
Unio atratus (t. 21, f. 18), Lea (as of Sw.).—U. auratus, Phil. 
(as Nata a. of Sw.) N. C. 3, Un. t. 4, f. 1.—U. Araucanus, Phil. 
N.C. 8, p. 50, Un. t. 4, f. 38. Oblong or ovate-oblong, inequilateral, 
not thick, convex or rather compressed, when adult greenish black, 
when young olivaceous-yellow changing to green anteriorly, nearly 
smooth, polished; front dorsal and yentr al edges convex, subparallel, 
the latter rising behind in an arch; both extremities a little attenu- 
ated, the front one more or less subangulated, the hinder one rounded ; 
beaks at about two-thirds the distance from the anterior end; um- 
bones eroded when mature, rayed with short narrow folds when 
young; nacre bluish; hinder teeth compressed, rugged, moderately 
elevated. 14...23. Chili. Usually thickly encrusted with ferru- 
ginous matter. I am not aware that Swainson ever published the 
species; tt is the “ Nata FrRaciuis, Sw.” of collectors, and the 
“N. arrata, Sw.” of Lea. 
Unio FontTaingeanus (t. 21, f. 27), D’Orb. Am. p. 605, t. 69, f. 6, 7. 
Obovate-rhomboid, rather compressed, thickish, blackish olive, not 
rayed, with close and numerous concentric wrinkles of increase ; 
ventral edge obliquely curving up behind; anterior side dilated, sub- 
angulated ae: rounded below, ligamental edge convex, a little 
ele wated ; hinder side very narrow, rounded ; umbones oblique ; um- 
bonal ridge not raised; the anterior slope ample, subretuse ; beaks 
eroded, when young with longitudinal corrugations ; ligament pro- 
duced; nacre bluish; primary teeth eiriniedl sublaminar: ; lateral 
teeth arcuated. 24...3. S. America. Is assuredly only a produced 
variety of the U. rotundus (Moricand Mem. Coq. p. 49, t. 4, f. 12, 3, 4 
(as Diplodon r. of Spix), of which the U. variabilis (p. 202) is stated 
to be the young. 
Unto auratus (t. 21, f. 29).— Nata a. Swainson, from types. 
Hlongated oblong, slightly arcuated, very imequilateral, thick, sub- 
ventricose (yet the surface a little flattened in the middle), of a some- 
what golden brown, not rayed, very coarsely wrinkled in a concentric 
direction ; ventral edge retuse, subparallel with the somewhat arcuated 
ligamental edge, curving upwards behind; posterior end short and 
abruptly rounded ; anterior extremity cradually rounded, the front 
margin arched and sloping; umbonal area simple; umbones not 
much raised, radiated with short divergent folds; lunule acutely 
edged; ligament tawny-brown, very long, prominent: inside wholly 

t The description of U. atratus, at page 199, was taken from 
Swainson’s U. (not Nata) fragilis, and consequently belongs to 
U. gracilis. 
