BIVALVIA. 63 
of the species of this genus; it resembles Myti/us decussatus, Montague, which Brown made 
the type of his genus Crene/la. This small and apparently rare shell of Lea is the only 
recorded fossil of the Eocene period belonging to the Mytiloid family from the American 
beds that I am acquainted with. A few small species from the Paris basin have been 
recently figured and described by M. Deshayes under the generic name of Crene//a. These 
French fossils have a more rounded form than any of our English species. 
Modiola is generally a marine genus, and it is found in most parts of the world, and 
at various depths ; two or three species have, however, been found in estuaries and where 
the water is sometimes fresh. Mr. McAndrew gives the vertical range of AZ. modiolus 
from the shore to 100 fathoms. 
1. Mopiona pupressa, J. Sowerby. ‘ab. XII, fig. 4. 
Mopiota peprEssA. J. Sow. Min. Conch., t. 8, three upper figs., 1812. 
—_ — Prestwich. Proc. Geol. Soc., May, 1847, p. 404. 
— — J. Morris. Catal. Brit. Foss., p. 211, 1854. 
— — Smith. Strat. Syst. Org. Foss. Lond. Clay, p. 2. 
Spec. Char. M. testa tenui, elongatd, obtusé-cuneatd aut irregulariter trapeziformi, com- 
pressd, levigatd ; umbonibus minutis, depressis, subterminalibus ; pedi-regione brevi, siphoni- 
regione elongata et latiore ; margine ventrali subrecta. 
Shell thin, elongate, obtusely wedge-shaped or irregularly trapeziform, compressed, 
smooth ; umbones small, depressed, and nearly terminal; pedal region short and rounded ; 
siphonal region long and broad ; ventral margin nearly straight. 
Length, 2 inches. 
Localities. Highgate (Sowerby), Sonning Hill (Prestwich). 
“This shell is two and a half as broad as it is long, and thin; the margin even and 
very regularly curved; it is altogether very flat, particularly so at the anterior (?) side ; 
the beaks are very slightly prominent and are rounded ; lines of growth faint ; external coat 
shining and pellucid, internal pearly. It is difficult to preserve, that being so extremely 
tender, the clay shrinks in drying, the shells crack and scale off in pieces, else the appear- 
ance of an epidermis is almost to be recognised.” (J. Sowerby.) 
Specimens of a shell strongly resembling this from Harwich, as also from the Barn Rock, 
near Bognor, and from Hollyport, are in the Museum of the Geological Society, but they 
are not perfect enough for fair determination. 
