BIVALVIA. 61 
1. Myrinus arrinis, J. Sowerby. Tab. XII, fig. 1, a—d. 
Mytiuus arrinis. J. Sow. Min. Conch., t. 532, fig. 1. 
_ — Morris. Catal. Brit. Foss., p. 215, 1854. 
Spec. Char. M. testa tenui, elongato-cuneatd vel obliqué-oblongatd, lavigatd, sub- 
carinatd ; margine dorsali subrecto ; intis margaritacea ; cardine edentato. 
Shell thin, elongately wedge-shaped, or obliquely oblong, smooth, subcarinated ; hinge- 
margin nearly straight ; interior pearly ; hinge without teeth. 
Length, 2 ches. 
Localities. 6, Barton; a, ¢, d, Colwell Bay, Headon Hill, Whitecliff Bay, Hordwell 
(Edwards). 
This species much resembles in outline some of the varieties of the common mussel, 
but it is carinated, that is to say, it is obtusely ridged at the most tumid or inflated part, 
and it is generally less rounded and more flattened in the siphonal region than in the 
recent British species; it is also very thin, and quite free from teeth at the umbo. 
Although not abundant, this fossil presents a good deal of variation in outline, the dorsal 
margin being much longer in some specimens than in others; but this kind of variability 
is common to the genus, and it may be seen in excess in the varieties of IZ. edulis. 
2. Mytinus stricitiatus, S. Wood. ‘Tab. XI, fig. 2, a, 6. 
Spec. Char. M. testa minima, tenui, ovato-cuneatd, obliqud, incurvd, inflata, radiatim 
striatd, striis oblusis conveais ; margine dorsali arcuatd, et crenulatd ; margine ventrali sub- 
ineurvad ; ared cardinali denticulatd. 
Shell small, thin, and ovately wedge-shaped, oblique, incurved, tumid, radiately striated ; 
striae obtuse, rounded; dorsal margin arched and strongly crenulated ; ventral margin 
slightly concave ; hinge-area denticulated. 
Length, } inch. 
Locality. Barton (Ldwards). 
Half a dozen specimens of this pretty little shell appear to offer good distinguishing 
characters. ‘The connexus was short, and situated within the rounded dorsal margin, but 
its action was over a fulcrum, and it was probably visible when the valves were closed. 
There are about half a dozen denticles in the hinge-area, variable in size, the two on the 
pedal side being the larger, and the dorsal margin outside the ligament is strongly den- 
ticulated, like Modiolarca. he strix, or rather ribs which ornament. the shell, are large, 
prominent, and rounded, not angular, and the spaces between them are as broad as the 
ribs ; an intermediate ray may be occasionally seen on the dorsal and siphonal regions as 
the shell enlarges. ‘This closely approaches J/odio/a, having a trifling extension beyond 
the umbo. 
