384 TESTACEOUS MOLLUSCA. 



Unio atratus (t. 21, f. 18), Lea fas of Sw.J. — U. auratus, Phil. 

 fas Naia a. of Sw.J N. C. 3, Un. t. 4, /. 1. — U. Araucanus, Phil. 

 $L C. 3, p. 50, Un. t. 4, /. 3. 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 ventral 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 ; urn- 

 bones eroded when mature, rayed with short narrow folds when 

 young ; nacre bluish ; hinder teeth compressed, rugged, moderately 

 elevated. 1^...2^. Chili. Usually thickly encrusted with ferru- 

 ginous matter. I am not aware that Swainson ever published the 

 species; it is the "Naia fragilis, Sw." of collectors, and the 

 " N. atrata, Sw." of Lea. 1 



Unio Fontaineanus (t. 21, f. 27), DVrb. Am. jr. 605, t. 69,/. 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 above, rounded below, ligamental edge convex, a little 

 elevated ; hinder side very narrow, rounded ; umbones oblique ; urn- 

 bonal ridge not raised ; the anterior slope ample, subretuse ; beaks 

 eroded, when young with longitudinal corrugations ; ligament pro- 

 duced; nacre bluish; primary teeth striated, sublaminar; lateral 

 teeth arcuated. 2£.<..3. S. America. Is assuredly only a produced 

 variety of the U. rotundus (Moricand Mem. Coq. p. 49, t. 4,/. 12, 3, 4 

 fas Diplodon r. of SpixJ, of which the U. variabilis (j;. 202) is stated 

 to be the young. 



Unio auratus (t. 21, f. 29). — Naia a. Swainson, from types. 

 Elongated oblong, slightly arcuated, very inequilateral, 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 

 abruptty rounded ; anterior extremity gradually 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 



1 The description of U. atratus, at page 199, was taken from 

 Swainson 's U. (not Naia) fragilis, and consequently belongs to 

 U. gracilis. 



