174 Mr. T. n. Withers on some 



siderahly above a riglit angle; tergo-lateral margin concave 

 in its upper part, the lower part being rounded and pro- 

 tuberant. Ba^i-lateral angle somewliat acute, with a slight, 

 narrow, square-edged extension formed by the projection of 

 the ridge extending from the apex t) this point ; this projec- 

 tion is extremely prominent in the scuta from the Chal'c 

 ]Marl (PI. VIII. fig. 4). The apico-basal ridge is very con- 

 spicuous, beiug formed of a single ridge in the Gault valves 

 and of two ridges in the Chalk Marl valves, and extends in a 

 strono'lv curved line from the apex about midway between 

 the outer margins ; it is narrow, being usually about half the 

 widtli of a zone of growth, aud. has perpendicular sides, or is, 

 as Darwin said, wall-sided ; where the transverse ridges 

 cross this ridge it is produced into slight prominences, 

 varving in prominence in different specimens; but in those 

 froin the Chalk ^larl the ridge is produced into sharp points. 

 A slight ridge extends from the apex near and parallel to tlie 

 upper part of the tergo-lateral margin, and from this ridge 

 the valve is inwardly rounded. Some valves are ridged 

 longitudinally, and others not, bat all those from the Chalk 

 ]Marl are strongly ridged longitudinally between the trans- 

 verse ridges. On the inner surface the occludent edge is 

 broad aud flat, is widest adjoining the top of the pit for the 

 adductor muscle, being there nearly half the width of the 

 valve, and is marked with growth-lines ; a deep triangular 

 furrow, marked with growth lines, lies near the tergal mar- 

 gin, and serves for the reception of the scutal angle of the 

 tergura ; just below tlie furrow and the flat occludent edge 

 there is a deep pit for the adductor muscle. 



Tergum (PL VII. fig. 18) subrhomboidal, almost flat trans- 

 versely, with a curved, narrow, wall-sided ridge like that of 

 the scutum, extending from the apex to the basal angle, 

 where it is produced ; apical portion much curved towards 

 the scuta. The apico-basal ridge is situated about one-third 

 the width of the valve from the carinal margin, is only very 

 slightly raised where crossed by the transverse ridges, which 

 are not so prominent as on the other valves ; in the valves 

 from the Chalk Marl the ridge is produced into prominent 

 sharp points. The upper carinal margin is slightly longer 

 than the lower, and makes with it almost a continuous 

 curve; occludent margin slightly concave, shorter than the 

 scutal margin. A portion of the valve along the occludent 

 margin is rounded aud protuberant to the extent to which 

 the valve was overlapped by the scutum ; this raised portion 

 is followed by a depression, which is bounded by a slight but 



