844 



MR. T. H. WITHERS ON 



point about one-third the length of the valve from the broken 

 apical portions in the larger valves (figs. 1, 3). Obviously these 

 ridges serve for the reception of the angle of the alee on the 

 adjacent lateral valves, and the space between the ridges is 

 marked, in the older specimens, with transverse lines. The 

 portion of the A^alve enclosed by the ridges is half as wide as the 

 adjacent parts of the valve in the specimen figured (fig. .3 5), but 

 in the valves figured (figs. 16, 2 b) it is wider. At the base of 

 the two ridges extending from the apex, and only in tlie largest 

 valve (fig. 1 b), a slight transverse ridge is formed by the 

 thickening of that part of the valve enclosed by the ridges. 

 From a point about one-third from the base of the valve, a small 

 extent of the inner surface, parallel to the outer margins, is 

 marked with lines which extend upw^ard, and bend abruptly 

 inwards towards the base of the ridge on either side ; these lines 

 are made by the ala? of the la,teral compartments which are 

 overlapped by this part of the valve. 



Lateral compartment (figs. 4-6) with an ala on the rostral 

 side, moderately convex transversely, and longitudinally almost 

 fiat, irregularly convex, and in one case bent in an elongately 

 ^-shaped curve; the whole valve is usually strongly bent 

 towards the rostrum, but one valve is strongly bent away from 

 the rostrum. Parietal portion very much wider than in the 

 carino-lateral compartment, as much as three times as Avide as 

 the widest part of the ala in one valve, and in others from two 

 to under one and a half times as wide. The two margins of the 

 ala form an obtuse angle, the upper margin is practically straight 

 and the lower somewhat concave, but their shape is influenced by 

 the degree of curvature of the valve ; the growth-lines on the ala 

 are closely set and extend obliquely upwards from the base, and 

 on reaching almost to the upper margin curve downwards to the 

 angle of the ala; on the lower margin, near the angle, a small 

 smooth portion is left just below where the growth-lines bend 

 downwards. On the inner surface, almost at the middle of the 

 parietal portion, a more or less prominent ridge extends from 

 the apex and dies out at a point opposite the angle of the ala ; 

 near the parietal margin the inner surface is marked with lines 

 which extend upwards, and on reaching a point just above the 

 base of the longitudinal ridge bend sharply inwards and down- 

 wards to meet its lower extremity; these lines are obviously 

 made by the ala of the carino-lateral compartment, the angle of 

 which abuts against the longitudinal ridge. Between the Tongi- 

 tudinal ridge and the upper margin of the ala the inner surface 

 is marked with indistinct and irregular transverse lines. 



Carino-lateral compartment (figs. 7-9) obtusely triangular in 

 general outline, with an ala on the rostral side. The whole valve 

 bent, especially in its apical half, toAvards the rostrum, is almost 

 flat transversely, and the parietal portion is much narrower than 

 that of the lateral compartment ; the two margins of the ala 



