BIVALVIA. 61 
ligament, but they extend along the whole line of hinge or dorsal area; the posterior 
side in some specimens is much rounded, so as to give a cylindrical form to the shell, 
while in others there is an angular slope on the posterior side, from a little beyond 
the termination of the hinge line to the ventral margin, which gives a greater width 
to that part, thereby producing a different form, and which, in consequence, were that 
a permanent character, might be considered a distinct species, but my specimens are 
exceedingly variable, so as to present no marked distinction between the two. 
5. Mopioua sERICEA, Bronn. Tab. VIII, fig. 3. 
Mopiona sERIcEA. Bronn. Ital. Tert. Geb., p. 122, No. 649, 1831. 
—_ — Phil. En. Moll. Sic., vol. i, p. 71, t. 5, fig. 14, 1836. 
— td - - vol. ii, p. 52, 1844. 
— a Nyst. Add. 4 la Faune. Conch., p. 444, No. 38, 1842. 
= HyaLINA. S. Wood. Catalogue, 1840. 
MyTILUs sERIcEus. Goldf. Pet., vol. ii, p. 179, pl. 131, fig. 12, a—e, 1841. 
— — Nyst. Conch. Foss. de Belg., p. 271, pl. 21, fig. 2, B—e, 1844. 
— — LE. Sism. Synop. Meth. An. Invert., p. 15, 1847. 
Bronn. Ergeb. meiner. naturh.-dkon Reise, vol. ii, p. 620, sec. Nyst. 
Spec. Char. Testa tenuissime, subhyalind, elliptico-ovatd, tumidd ; valde inequilatera ; 
striaté, striolis eailibus confertis ; umbonibus prominentibus recurvis ; margine ventrali 
subrecto, tenuissime crenulato. 
Shell thin delicate, subhyaline, of an elliptical or ovate form, tumid; very inequi- 
lateral ; covered externally with numerous close-set, extremely fine, radiating striz ; 
umbo projecting beyond the anterior side; ventral margin nearly straight and 
finely crenulated. 
Locality. Cor. Crag, Ramsholt and Sutton. 
It is very difficult to obtain specimens of this species in perfection, on account of 
its extreme thinness, although they appeared at Ramsholt to be by no means rare, but 
they are generally more or less broken or distorted. 
This beautiful species is of an ovate or elliptical form, very tumid, excessively 
thin and semitransparent ; it is covered over its whole surface with extremely 
fine radiating striz, crossed occasionally by irregular lines of growth, but not 
regularly decussated ; the ligamental area extends about half the distance of the 
dorsal portion, its termination forming a very obtuse angle ; the umbo is terminal and 
somewhat prominent, curved, and slightly projecting beyond the anterior margin. 
This appears at present to be known only as a fossil. My largest specimen ~ 
measures an inch and a quarter in its longest diameter. 
