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Eocene Fossils of the United States 217 
Mactra decisa, Con.; Foss. Shells of ert Form., p. 42, Oct. 
1833. 
M. dentata, Lea; Cont. to Geol., p. AM, Plate I, fig: 9% Dec. 
1833. 
Claiborne, we 4 
This very rare species has not been found entire. I have only 
one fragment, and the Ringe of this is perfect enough to exhibit 
the fissure in the hinge line under the apex, which characterizes 
the genus Scissodesma. 
Macrra paritis. Triangular, equilateral; smooth and polish- 
ed; both ends depressed and striated; umbonial slope slightly 
folded; anterior angle with an impressed line ; cavity capacious. 
_(Plate I, fig. 6 
Mactra parilis, Con.; Foss. Shells of 'Tert. Form., p. 42, Oct. 
1833. 
M. pygmea ? Lea; Cont. to Geol., p. 44, Plate I, fig. 11, Dee. 
1833. 
Claiborne, Alabama. 
This species has some resemblance in outline to the M. late- 
ralis of Say, but its specific character is very different. It probably 
belongs to the genus Mulinia. ; 
Macrra prerenuis. Subtriangular, compressed, equilateral, 
thin and fragile ; umbonial slope submarginal, subrectilinear, cari- 
nated; beaks slightly prominent ; posterior slope with two prom- 
inent fuse lines and obliquely rugose ; surface of the valves with 
very minute concentric closely arranged lines. (Plate II, fig. 4.) 
Mactra pretenuis, Con.; Foss. Shells of -Tert. Form., p. 42. 
Claiborne, Alabama. 
This shell is very rare. It has somewhat the outline of M. 
delumbis, of the Miocene, but is a much smaller and very distinct 
species. It has the cardinal fissure which would constitute it a 
member of the genus Scissodesma. 
TriquetTra, Con. 
This genus differs from Mactra and Mesodesma in vide a 
less profound and much smaller cardinal fosset, which does not 
extend more than half the width of the cardinal plate, which is 
broad and thick. The substance of the shell is thicker than in 
Mactra, and the anterior muscular impression more profound. 
Exteriorly this genus is distinguished by a triangular outline, 
Sxconp Series, Vol. I, No. 2.—March, 1846. 28 
