BIVALVIA. 61 



1. Mytilus affinis, J. Sowerby. Tab. XII, fig. 1, a — d. 



Mytimjs affinis. J. Soiv. Min. Conch., t. 532, fig. 1. 



— — Morris. Catal. Brit. Foss., p. 215, 1854. 



Spec. Char. M. testa tenui, elongato-cuneatd vel oblique -oblongata, laevigata, sub- 

 carinatd ; maryine dorsali subrecto ; intiis maryaritacea ; car dine cdentato. 



Shell thin, clongatcly wedge-shaped, or obliquely oblong, smooth, subcarinated ; hinge- 

 margin nearly straight ; interior pearly ; hinge without teeth. 



Lenyth, 2 inches. 



Localities, b, Barton ; a, c, d, Colwell Bay, Headon Hill, Whitccliff 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 M. edulis. 



2. Mytilus strigillatus, S. Wood. Tab. XII, fig. 2, a, b. 



Spec. Char. M. testa minima, tenui, ovato-cuneatd, obliyud, incurvd, inflatd, radiatim 

 striata, striis obtusis convexis ; maryine dorsali arcuatd, ct crenidatd ; maryine venlrali sub- 

 incurvd ; area cardinali denticulatd. 



Shell small, thin, and ovately wedge-shaped, oblique, incurved, tumid, radiately striated ; 

 stria? obtuse, rounded ; dorsal margin arched and strongly crenulated ; ventral margin 

 slightly concave ; hinge-area denticulated. 



Lenyth, \ inch. 



Locality. Barton {Edwards). 



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. The strise, 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 Modiola, having a trifling extension beyond 

 the umbo. 



