MVALVIA. 63 



of the species of this genus ; it resembles Mglihi* decussatus, Montague, which Brown made 

 the type of his genus Crcnella. 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 Crenella. 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. Mc Andrew gives the vertical range of M. modiolus 

 from the shore to 100 fathoms. 



1. Modiola depressa, J. Sowerbg. Tab. XII, fig. 4. 



Modiola. depressa. J. Sow. Min. Conch., t. 8, three upper figs., 1812. 



— — Prestwich. Proc. Geol. Soc., May, 1847, p. 404. 



— — /. Morris. Catal. Brit. Foss., p. 211, 1854. 



— — Smith. Strat. Syst. Org. Foss. Lond. Clay, p. 2. 



Spec. Char. M. testa tenui, elongatd, obtuse-cuneatd aut irregulariter trapeziformi, com- 

 pressd, lavigatd; umbonibus minutis, depressis, subterminalibus ; pedi-regione brcvi, siphoni- 

 regione elongatd et latiore ; margine ventrali subrecld. 



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 {Sowerbg), 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." (/. Sowerbg.) 



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. 



