92 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Genus UNIO, RETZIUS. 



Synon.'Unlo, RF.TZ. Diss. Phys. 1788, 16. BRITG. Jour. d'Hist. Nat. 1792, and Encyo. Meth. I, tab. 247. Cuv. 

 Tab. Elem. 427. LAMK. Prod. 1799, 7, and Syst. 1801, 114. 



Mya, HUMPHREY, Mus. Coll. 1797, 59 (not Lixx.). 



Limnium, OKES, Lehrb. d. Naturg. Ill, 1815, and 1821, Naturg. f. Schnl. 651, 8. 



Klliplo, RAP. Jour. Phys. torn. 88, 1819, 426, and 1820 Monogr. Bivalves of the Ohio. . . . 



Margarita (part), LEA, Trans. Am. Phila. Soo. VI (u. s.), 1 (not LEACH, 1819). 



Cunicula, SWAINSON, Malae. 1840, 267, and 378. 



Margaron (part), LEA, Synop. Naid. (3d ed.), 1852, p. xvii. 

 Elym. Unio, a pearl. 

 Type. Afya pectorum, LINN. 



Shell variable in form, usually oval, elongate or oblong ; surface covered with a 

 brownish or olivaceous epidermis, sometimes striped with greenish and olive bands. 

 Beaks often eroded. Nacre white, yellowish, flesh-colored, or purple. Hinge 

 variable, generally with two anterior teeth in one valve, and one in the other, or 

 two in each ; posterior teeth elongate and laminar, usually two in one valve and one 

 in the other. 



Animal, in the typical species, with gills free from the abdominal sac, their 

 posterior extremity attached to the mantle ; eggs in the female filling the whole 

 extent of the outer gill ; mantle fringed at both syphonal openings. (Agassiz.) 



As above restricted, it will be rather difficult to determine the geological range 

 of the genus Uhio, since some of its more important distinctive characters are such 

 as belong to the softer parts of the animal only. The oldest known species appa- 

 rently belonging to this genus have been described from the Jurassic rocks, though 

 it is somewhat doubtful whether these are true Uniones. It was formerly supposed 

 that some Carboniferous and Devonian shells belonged to this genus, but they are 

 now all known to belong to Cardinia, Carbonocola, and other extinct groups. 

 Several species apparently presenting the characters of this group have been de- 

 scribed from the Wealden beds, but this type of life seems not to have been very 

 generally distributed over the world until the Tertiary epoch; and it undoubtedly 

 attains its greatest development at the present time, and in the streams of this 

 country. (Type 192.) 



Unio nucalis. 



(PLATE III, Fig. 13, a, b, c.) 

 Unio nucalis, MEEK & HAYDEN, Proceed. Aead. Nat. Sci. Phila. March, 1858, p. 52. 



Shell longitudinally oval, gibbous in the central and nmbonal regions. Anterior extremity rather narrowly 

 rounded ; base semi-oval, sometimes rather prominent near the middle ; posterior end subtruncate, or forming a 

 regular curve from above, and rather narrowly rounded below. Beaks moderately depressed, located about half 

 way between the middle and the anterior end, not eroded ; posterior umbonal slopes prominently rounded. 

 Surface marked by fine obscure concentric lines, and more or less distinct marks of growth ; tbe latter becoming 

 small, and very regular wrinkles on the beaks. 



Length, about 1.63 inch ; height, 1.05 inch; breadth, 0.82 inch. 



1 A number of the names mentioned in connection with the family Unionidee are doubtless only 

 synonyms of Unio; but until the limits of this genus have been more satisfactorily determined, it, is 

 probably better to include only those more exactly synonymous with the genus as restricted to the 

 typical forms. 



