180 Mr. T. H. Withers on some 



tergo-lateral margin usually concave in its upper part, and 

 convex below. The a pi co- basal ridge is very prominent, 

 rather broad, about as broad as a zone of growth, slightly 

 rounded at its summit, lias steeply sloping sides, and extends 

 in a curved line rather nearer to the tergo-lateral margin. 

 Apico-basal ridge not at all produced where crossed by the 

 prominent transverse ridges ; the transverse ridges are 

 strongly raised, equally spaced, and bend downwards and 

 are sliglitly thickened at the occludeut margin. A slight 

 ridge can be seen extending from the apex to the tergo- 

 lateral angle, and from this ridge the valve at the upper 

 part of the tergo-lateral margin is inwardly rounded. On 

 the inner surface the occludent edge is much thickened, 

 broad and flat, is widest adjoining tlie top of the pit for the 

 adductor muscle, being there more than two-thirds the width 

 of the valve ; it is marked with growth-lines. The inner 

 margin of the occludent edge is considerably raised and over- 

 hangs the subtriangular depression for the reception of the 

 scutal angle of the tergum. 



l^ergum (PI. VII. fig. 13) subrhomboidal, somewhat convex 

 transversely, with a curved ridge like that of the scutum, 

 from which the sides of valve slope steeply, extending from 

 the apex to the basal angle, but not projecting beyond it ; 

 apical portion of the valve slightly to moderately curved 

 towards the scuta. The apico-basal ridge is situated almost 

 in a median line, and is not produced where crossed by the 

 transverse ridges. Upper carinal margin usually slightly 

 shorter than the lower, both of which meet in a well-defined 

 angle ; occludent margin usually shorter than the scutal 

 margin, and of about the same length as the upper carinal 

 margin. A portion of the valve along the occludent margin 

 is rounded and protuberant, and on its inner margin is 

 bounded by a depressicm ; about midway between the de- 

 pression and the apico-basal ridge, slight indications can 

 be seen in some specimens of an indistinct ridge, evidently 

 homologous with that seen in the terga of P. rlgldus. 



Upper latus (PI. YII. fig. 12) shaped like a very acute- 

 angled isosceles triangle. External surface marked with 

 several raised, regularly spaced, and prominent transverse 

 ridges, slightly upturned at the lateral margins ; the inner 

 lateral margins have a serrated appearance owing to the promi- 

 nence of these outer ridges. On the inner surface the growth- 

 lines are obliquely upturned, and meet on a very prominently 

 raised, sliarp-edged ridge, which extends to the apex from 

 a point just above one-third the length of the valve from the 

 base. 



