CONCHOLOGY. 



which are obsolete in that part; within 

 bluish-white, margin white ; cavity of the 

 umbo not distinctly impressed by the ex- 

 ternal undulations; tooth compressed; 

 oblique, nearly parallel with the posteri- 

 or slope, and terminating abruptly be- 

 hind. 



Length, exclusive of the umbo, one 

 inch and one-fourth ; breadth, two inches 

 and a half. 



Journ. Acad. Nat. Sci ences. 



The inner margin is of a chalky white- 

 ness, in this respect resembling Anodonta 

 JUarginata. It was found by Mr. Isaac 

 Lea, in the river 



2. U. Undulata. 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, decorticat- 

 ed, and with four or five large, obtuse, dis- 

 tant, undulations, disappearing towards 

 the basal margin : within bluish-white, 

 cavity deep ; teeth one in each valve, 

 thick and strong ; that of the left valve 

 crenated, of the right valve somewhat 

 bifid, and gradually sloping to the hinge 

 margin. 



Length, three-fifths of an inch: breadth, 

 nine-tenths of an inch. 



Plate 3. fig. 3. 



Found in the Delaware and Schuylkill 

 rivers, but is rather rare, and resembles 

 Jlnodonta Undulata. 



This genus, in conjunction with Dipsas 

 of Leach, will complete the chain of con- 

 nection between the two genera Unio 

 and Jlnodonta. It corresponds With these 

 genera in the number of its cicatrices, 

 but is separable from Anodonta, by its 

 primary tooth ; from Unio by being des- 

 titute of the lamelliform teeth ; and from 

 Dipsas, also by the last mentioned charac- 

 ter, as well as by the presence of a pri- 

 mary tooth, which is wanting in that genus. 



This new genus we proposed in the 

 former editions of this work, when de- 

 scribing the Undulata, under the name of 

 Monodonta ; but as the same term has 

 been applied to a genus of univalves, I 

 have substituted that of Alasmodonta. 



GErfUS AKODOSTTA. 



Shell transverse, with three obsolete 

 muscular impressions, hinge simple, des- 

 titute of teeth. 



Qbs. 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 perlaceous ; 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. Jlnatinus 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 ; anterior 

 margin fuscous ; beaks nearer the poste- 

 rior end ; within bluish white, edged 

 with whitish. 



Length, one inch and a half: breadth, 

 two inches. 



Plate 3. fig. 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 radiate 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. 



Length, nearly half an inch: breadth, 

 nearly seven-tenths of an inch. 



Plate 3. fig. 6. 



This species is perhaps rare : it does 

 not exactly agree in all its characters 

 with the genus, but approaches nearer 

 it than to any other ; it resembles Alas- 

 modonta Undulata, for the young of 

 which it might readily be mistaken. 



GENUS CYCLAS. 



Shell almost orbicular, or a little trans- 

 verse, without fold on the anterior mar- 

 gin ; two or three primary teeth/ and 

 lateral, remote, lamelliform ones on each 

 side. 



Observ. The shells of which this ge-. 

 nus is composed were formerly placed 



