18G Mr. T. H. Withers on some 



set, anfl have their edges somewhat rounded and flattened. 

 The longitudinal ridges are also somewhat flattened, and on 

 the scuta and terga have a wavy appearance and radiate 

 from the apico-basal ridge. 



Carina (PI. VII. fig. 4) semicylindrical, widening gradually 

 from the apex to the basal margin, slightly to moderately 

 bowed inwards, strongly convex transversely, not carinate, 

 basal margin almost straight. Outer surface ornamented 

 with a number of prominent, but somewhat flattened trans- 

 verse ridges, crossed by fine, rounded, closely set, longitudinal 

 ridges. The apical portion projected freely for less than a 

 third of the length of the valve, and the portion is marked 

 with growth-lines which extend from the basal angles and 

 meet in a rounded angle below the apex ; the inner lateral 

 edges of the valve are somewhat thickened for about one- 

 fourth the width of the valve. 



Rostrum (PI. VII. fig. 1) semiconical, smaller and propor- 

 tionally wider than the carina, widening rapidly from the 

 apex to the basal margin, moderately bowed inwards, strongly 

 convex transverseh^, basal margin concave. Outer surface 

 ornamented similarly to the carina. On the inner suiface 

 the lateral edges of the valve are thickened, the median third 

 of the valve forming a deep hollow between ; the apical 

 half of the valve projected freely, and this part is marked 

 with growth-lines which extend from the basal angles and 

 meet in a rounded angle below the apex. 



Scutum (PI. VII. fig. 2) elongately triangular, proportion- 

 ally narrow, almost flat transversely, apical portion much 

 bowed towards thetergum, narrow, and acuminate; occludent 

 margin strongly convex; basal margin less than half the 

 length of the occludent margin, and forming with it an angle 

 slightly less than 90° ; tergo-lateral margin strongly concave 

 in its upper part, its lower part being rounded and somewhat 

 protuberant. Basi-lateral angle, where the apico-basal ridge 

 slightly projects, is obliquely truncated. The apico-basal 

 ridge extends in a strongly curved line from the apex, rather 

 nearer to the tergo-lateral margin ; it is flatly rounded 

 transversely, has steep, but not perpendicular sides, and is 

 more than twice as wide as a zone of growth. Along the 

 tergal margin the valve is inwardly rounded, but there does 

 not appear to be any trace of a ridge. Outer surface orna- 

 mented with a number of prominent transverse ridges, the 

 interspaces of which are marked with raised transverse lines ; 

 the transverse ridges are crossed by fine, wavy, longitudinal 

 ridges radiating from the apico-basal ridge. On the inner 



