54 



LAMELLIBRANCHIATA. 



MaRTESIA (?) CARINIFERA. (N. Sp.) 

 Plate IX, figure 7. 



Shell tumid and ventricose in front, narrow and attenuate behind ; 

 about one-third longer than high. The thickness at the anterior end 

 equals or slightly exceeds the height, and the posterior compression is 

 much greater in a lateral than in a horizontal direction. 



The superior border is straight, but slopes gently downwards to the 

 posterior end ; the umbonal region is swollen and the antero-dorsal margin 

 is raised and rounded. The beaks themselves are anterior, terminal, 

 prominent, incurved and apijroximating. The lower half of the anterior 

 margin is truncate obliquely, though almost horizontally ; ils base is 

 roimded, and above the middle it seems to have been produced into a 

 more or less rounded lobe on each side, which extend upwards so as to 

 just touch each other immediately below the beaks. The edges of the 

 valves at this end are a little broken in the only specimen collected, so 

 that the outline of the anterior extremity cannot be very clearly ascer- 

 tained. The pedal opening or anterior gap is large and Avide, it appears to 

 have been nearly diamond-shape, but the upper half probably had concave 

 sides, and was shorter than the lower. The height and width of the 

 pedal opening were apparently about equal. 



The posterior margin is obliquely subtruncate, bluntly angular and 

 pointed below, less so above. The ventral border is straight, but trends 

 slightly upwards towards the posterior end ; its tennination in front is 

 al?ruptly rounded, and bluntly angular behind. 



On each valve a slightly flexuous raised ridge runs obliquelj^ from be- 

 hind the beaks to] the posterior end of the basal margin, and separates a 

 small, excavated and nearlj' triangular posterior area, from the central 

 part of the shell. The middle of the valves is also divided by two trans- 

 versely oblique and slightl}^ divergent grooves (which pass from the beaks 

 to the ventral margin) into three unequal and differently shaped spaces. 

 Of these, the posterior is the largest, the centre one the smallest, while 

 the anterior space is of medium size. The latter is marked near its front 

 boundary by two raised lines, which pass from the beaks to the base, and 

 in so doing cut or divide oft', as it were, the two lobes at the front end 

 from the main body of the shell. 



A small portion of the test is preserved on one of the valves, and under 



