SPECIES.
1. U. Crams. 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, 41 2 41 2 2.
Plate 1. fig. 8.
?uscblis brevoir, admodum crassus, ex in terna parte aubroseus, cardine incisuris mi nutia exasperate. Lister. Conch. tab. 150.
,cg. 5.
Encyc, Method. vol. 63. tab. 249. ,fig. 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 sub oval, 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. Purpuretts. 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.
Length, one inch and a half : breadth, two and a quarter.
Plate 3. fig. 2.
.11Iuscules latior, subfuscus, Ceruleis neis Radiates. Lister. Conch. tab. 152.