133 
not so far backward as in the present shell, and the remarkable 
membrane, induced me to describe it as new. It occurred on the 
rocky shore of the river, and appears to be rare. 
Unio purpuriatus. — Transversely oval, slightly oblong, in 
some specimens with a little tendency towards ovate ; dirty yellow- 
ish or fuscous, obscurely radiate with blackish-green : beaks, 
behind the middle, hardly raised : anterior edge rounded, or rather 
composed of two nearly rectilinear lines : posterior margin rounded : 
hinge margin regularly rounded : cavity of the hinge membranes 
(behind the beaks) narrow, but very obvious : basal margin arquated, 
a little compressed in the middle, and sometimes almost contracted 
in that part : within, purple ; margin livid ; anterior submargin 
iridescent : primary teeth nearly direct, rather thick, striated ; 
lateral teeth hardly extending beyond the sinus of the hinge 
margin. 
Length, one inch and three-fifths ; breadth, three inches and 
one-tenth. Convexity, over nine-tenths of an inch. Yar. a. 
Within white : Yar. h. 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 Y era 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 Mexican analogue of the pur- 
pureus. 
Unio metallicus. — Transversely ovate, very slightly oblong : 
dull yellowish, tinged with coppery, particularly on the umbones : 
' beaks slightly elevated, not decorticated, undulated ; the undular 
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 
