CONCHOLOGY. 



irregular tooth, and anterior lamelliform, 

 elongated one in one valve, inserted be- 

 tween two corresponding teeth and lami- 

 nae of the other. 



Obs. The shells of this genus inhabit 

 fresh water : they were placed by Lin- 

 naeus with the myae t from which they dif- 

 fer in very essential particulars ; as is ob- 

 vious from the number, situation, and 

 figure of the teeth, and the organization 

 and habits of the included animal. 



1. U. Crassus. Shell varying in form 

 and surface ; remarkably thick and pon- 

 derous ; epidermis horn-colour, different 

 shades of brown or black ; beaks carious, 

 often much eroded, pure parlaceous, or 

 silvery white, more prominent as the shell 

 approaches an orbicular form ; primary 

 teeth, not very oblique. The dimen- 

 sions of three specimens were as fol- 

 low : 



Length, 3 inches; breadth, 4 

 2 4* 



2 2. 



Plate 1. fig. 8. 



JWuscblis brevoir, admodum crassus, ex in- 

 terna parte subroseus, cardine incisuris mi- 

 nutis exasperato. Lister. Conch, tab. 150. 

 *.* 



Encyc. Method, vol. 63. tab. 249. ^. 1. 



By these dimensions it will appear to 

 vary very considerably, being a regular 

 oval ; much elongated ; ovate or nearly or- 

 bicular ; sometimes with two or three 

 longitudinal or oblique waves ; rarely tu- 

 berculate ; within parlaceous or red pur- 

 ple ; teeth very thick, crenated, and re- 

 sembling those of the next species. It is 

 very probable that we have here included 

 several distinct species, but at present we 

 are not sufficiently well acquainted with 

 the inhabitants to separate them. Gmelin 

 refers to Lister's figure for his Mytillus 

 Cygneus, but we can discover no resem- 

 blance between them. 



Found plentifully in the river Ohio and 

 its tributary streams. 



2. U. Plicata. Shell suboval, thick and 

 ponderous ; valves with two or three 

 more conspicuous undulations, which are 

 profound, very oblique, continued to the 

 anterior basal edge, and not arising from 

 the umbo ; umbo decorticated, some- 

 times much eroded ; within parlaceous, 

 distinctly impressed by the undulations. 



This species may be distinguished from 

 any of the numerous varieties of the pre- 

 ceding species by the oblique direction 

 of the undulations, of which those nearest 



the base are largest and seem to originate 

 behind the beaks. 



It was found by Mr. Lesueur in Lake 

 Erie, and was communicated by him un- 

 der the above name. 



3. U. Pitrpureus. Shell sub-oval, some- 

 what compressed, with smaller wrinkles 

 placed between larger ones, colour dark 

 brown ; beaks placed nearer one end, 

 very carious, not prominent, generally 

 the epidermis and pearly strata are re- 

 moved, exhibiting a wax-yellow ground : 

 within reddish purple, varied with green; 

 no cavity under the beak : teeth resemble 

 the preceding. 



Length, one inch and five-eighths : 

 breadth, two inches and four-fifths. 



Plate 3. fig. 1. 



This species is more numerous in the 

 rivers Delaware and Schuylkill than any 

 other of the genus : in the rivers of the 

 southern states it arrives at a more con- 

 siderable magnitude, measuring some- 

 times four inches, or more, in breadth. 

 These large specimens, and sometimes 

 the smaller ones, are a little shorten- 

 ed on the base opposite to the lamellar 

 teeth. 



4. U. Ovatus. Shell sub-ovate, convex, 

 not remarkably thick, horn colour, not 

 radiated ; flattened and fuscous on the an- 

 terior margin ; beaks decorated, placed 

 nearer central ; umbo prominent ; with- 

 in parlaceous ; cavity of the beaks capa- 

 cious; primary teeth very oblique, almost 

 parallel to the posterior margin and much 

 compressed. 



Length, three inches ; breadth, four 

 inches. 



Inhabits the Ohio river and its tributary 

 streams. 



Encyc. Method, vol. 63. tab. 248. fig. 5. 



Plate 2. fig. 7. 



5. U. Cariosus. Shell moderately thick, 

 much longer before, and shorter behind 

 the beaks ; olive green, sometimes radi- 

 ate with green, and with fine interrupted 

 wrinkles placed in longitudinal rows, but 

 usually the green radii are wanting, or 

 only visible in the anterior margin, and 

 the wrinkled radii indistinct; in older 

 shells the middle of the base is a little 

 shortened ; beaks somewhat prominent, 

 rather distant, carious, exposing a wax- 

 yellow surface ; concavity bluish-white, 

 teeth resembling those of the preceding 

 species : but the primary ones are not so 

 much compressed or oblique ; they are 

 often sub -conic and crenate. 



