CONCHOLOGY. 



kJed ; beaks not prominent, placed very 

 far back and decorticated ; base almost 

 rectilinear; hinge margin remarkably ob- 

 lique, rising near the termination of the 

 cartilage into an alated projection, and 

 forming almost a right angle with the in- 

 terior slope, which is nearly equal in 

 length ; within red-purple, often with nu- 

 merous tubercles, which upon the gaping 

 limb are confluent ; cicatrices very 

 rough ; teeth crenate, the external lami- 

 nated one obsolete, only one in each 

 valve being very perceptible. 



Length, including the projection, three 

 inches and/our fifths ; breadth, five inch- 

 es and a half. 



Plate 4. fig. 2. 



Mr. Lesueur found this species in Lake 

 Erie, very thin and fragile. 



The specimen from which this descrip- 

 tion is taken, and also tin- next species, 

 were purchased by Mr. Joseph Watson, 

 at the sale of thte late Professor Bar- 

 ton's collection, and by him presented 

 to the Academy of Natural Sciences of 

 Philadelphia There is also a specimen in 

 the Museum of Mr. Peale. 



8. U. Cylindricus. Shell very thick, 

 sub-cylindrical, emarginate before, pale 

 horn-colour varied with greenish ; hinge 

 margin undulated obliquely across the 

 wrinkles, rough on each side, rectiline- 

 ar and parallel to the base : beaks broad 

 and prominent, with a deeply impressed 

 posterior lunule ; within pearlaceous, 

 teeth thick and crenate ; cavity of the 

 beaks very deep. 



Length, one inch and three-tenths ; 

 breadth, three inches and one fifth. 



Plate 4. fig. 3 



From the collection of the late profes- 

 sor Barton ; it is said to be found in the 

 river Wabash. 



9. U. Undnlata. Shell thin, convex, sub- 

 oval, greenish or olivaceous, with ob- 

 tuse concentric wrinkles and radiate with 

 green, a little uneven before : beaks pro- 

 minent, acute, approximate, decorticated, 

 and with four or five large, obtuse, dis- 

 tant, undulations, disappearing towards 

 the basal margin : within bluish-white, 

 impressed by the external waves ; beak 

 cavity deep; teeth one in each valve, 

 thick and strong; that of the left valve 

 crenated ; of the right valve somewhat 

 bifid, and elongated before, and gradual- 

 ly losing itself in the hinge margin. 



Length, three-fifths of an inch; breadth, 

 nine-tenths of an inch. 



Plate 3. fig. 3. 



Found in the Delaware river, but is 

 rather rare, and resembles Anodonta Un- 

 dulata. 



This shell certainly does not belong to 

 the genus Uuio, as we have here restrain- 

 ed it ; in our manuscripts we have con- 

 structed a genus for it, under the name of 

 Monodtmta, which will be properly pla- 

 ced between Unio and Anodonta. 



G K VfS AXODONTA. 



Shell transverse, with three obsolete 

 muscular impressions, hinge simple, des- 

 titute of teeth. 



Obs. The shells which constitute this 

 genus were arranged by Linnaeus and 

 many other writers, under the genus 

 Mytillus. 



1. A. Cataracta. Shell thin, fragile, 

 translucent, oblong oval, convex, cover- 

 ed with a green olive, radiated, epider- 

 mis, within pearlaceous ; beaks nearer 

 central, frontal margin brown. 



Length, two inches and two-fifths: 

 breadth, four inches and an half : conca- 

 vity of one valve nearly seven-eighths of 

 an inch. 



Plate 3. fig. 4. 



This large muscle occurs in lakes, mill- 

 dams, &c. and bears some resemblance 

 to the A Anatinu* of Europe. 



Found by Mr. I. Lukens, in the deep 

 part of a mill-dam. 



2. A. Marginata. Shell very thin, fra- 

 gile, somewhat compressed, translucent, 

 subovate ; epidermis green olive, paler 

 on the disk and greener before ; anterfor 

 margin fuscus ; beaks nearer the poste- 

 rior end ; within bluish white, edged 

 with whitish. 



Length, one inch and a half : breadth, 

 two inches and four-fifths. 



Plate 3. fifr 3. 



Resembles the preceding, but is more 

 ovate, and the beaks are placed much 

 further back; it is very common in our 

 rivers. 



3. A. Undulata. Shell thin, fragile, 

 convex, olivaceous, obscurely radi&te and 

 obtusely wrinkled ; umbo prominent, de- 

 corticated, with four or five obtuse un- 

 dulations disappearing on the disk. In 

 the right valve, immediately under the 

 beak, the margin is curved inwards for 

 the reception of a corresponding margi- 

 nal projection of the opposite valve. 



