The Invertebrate Fauna of the Uitenhage Scries. 115 



to the frontal margin. It seems doubtful from Eichwald's figures 

 whether a single form alone is represented, for it is difficult to 

 believe that the originals of pi. xiii., fig. 12, pi. xiv., fig. 1, and 

 pi. xiv., fig. 3 can belong to one species. Other points in which 

 these differ from T. vau are the great extent of the frontal ribs, the 

 horizontal position of these, and the persistent transverse ornaments 

 on the area. Eichwald's description and figures leave the affinities 

 of T. doroschini obscure, but I think it improbable that it is related 

 to the group of T. vau. 



TRIGONIA STOWI sp. nov. 

 Plate VI., figs. 4, 4a, 46, 5 ; VII., fig. 1. 



Description. The shell is of very elongated form, moderately 

 inflated, anteriorly pointed, posteriorly very much produced, and 

 slightly gaping at the siphonal margin. The umbones are situated 

 at some point lying between one-quarter and one-third of the 

 shell's total length from the anterior extremity ; they are well 

 incurved and slightly recurved. The umbonal region is fairly 

 prominent and inflated. Posteriorly to the umbo, the very long 

 cardinal margin forms a straight or very gently concave outline and 

 passes posteriorly by a curve into the short, convex siphonal margin. 

 Anteriorly to the umbo, the valve margin slopes downwards with 

 straight profile to the projecting anterior extremity, where it forms a 

 sub-angular or sharply curved junction with the lower margin ; the 

 latter forms a gentle and evenly convex profile as it is traced between 

 the anterior and posterior extremities of the shell. 



At no stage of growth is a marginal carina developed; in the 



youthful stage its place is represented by a blunt carinal fold which 



passes with advancing growth into a still broader and less well 



defined rounded fold of the valve. At the siphonal end of an adult 



individual this loses definition and the valve becomes evenly convex. 



In the youthful shell, until the valve attains a height of about 



8 mm. measured from the umbonal apex, the ornamentation 



consists of numerous, crowded linear ribs which are parallel to the 



lower margin and extend from the frontal border across the flank ; 



they pass over the carinal angle and across the area. Up to a 



distance of between 5 mm. and 10 mm. from the umbonal apex these 



ribs extend across the escutcheon also, and terminate either at the 



cardinal margin or just before reaching it. Subsequently they cross 



the area though not the escutcheon, until a distance of about 10 or 



12 mm. from the umbonal apex is reached, after which they occupy 



the flank only, terminating at the carinal angle, while the area subse- 



