133 



iOt so far backward as in the present shell, and the remarkable 

 iiembrane, induced me to describe it as new. It occurred on the 

 ocky shore of the river, and appears to be rare. 



Unio purpuriatus. — Transversely oval, slightly oblong, in 

 ome specimens with a little tendency towards ovate ; dirty yellow- 

 sh or fuscous, obscurely radiate with blackish-green : beaks, 

 tehiud the middle, hardly raised : anterior edge rounded, or rather 

 lomposed of two nearly rectilinear lines : posterior margin rounded: 

 linge margin regularly rounded : cavity of the hinge membranes 

 behind the beaks) narrow, but very obvious : basal margin arquated, 

 I little compressed in the middle, and sometimes almost contracted 

 n that part : within, purple ; margin livid ; anterior submargin 

 ridescent : primary teeth nearly direct, rather thick, striated • 

 ateral teeth hardly extending beyond the sinus of the hinge 

 margin. 



Length, one inch and three-fifths ; breadth, three inches and 

 )ne-tenth. Convexity, over nine-tenths of an inch. Var. a. 

 Within white : Var. b. Within dull yellowish. Inhabits Mexico. 



For this species I am indebted to Mr. Maclure. It was found 

 by William Bennet in a stream a few leagues from Vera Cruz, 

 where it is abundant. It resembles U. purpureus, nob., but dif- 

 fers in having the teeth more direct; in the lateral teeth being 

 shorter, with respect to the sinus of the hinge margin ; in having 

 the greatest breadth at the middle of the anterior margin, &c. 



In the young and middle-aged specimens the radii are very 

 distinct, but are obsolete in old specimens. The umbones are 

 widely decorticated in age, but seldom are they so deeply eroded 

 as to disclose the waxen colored Stratum. 



It may be considered as the ]\Iexiean analogue of the p«r- 

 imreus. 



Unio metallicus. — Transversely ovate, veiy slightly oblong : 

 dull yellowish, tinged with coppeiy, particularly on the umbones : 

 beaks slightly elevated, not decorticated, undulated ; the undula- 

 tions obsolete in their middles, and not extending on the umbo 

 placed far back : hinge margin prominently arquated : posterior 

 margin very short, rounded : anterior margin rounded : basal mar- 

 gin subrectilinear : within liver color : iridescent on the anterior 

 margin : cardinal teeth rather long, somewhat tuberculous, not 

 very prominent ; that of the right valve not abruptly terminated 



