116 Annals of the South African Museum. 



quently remains smooth. In the space between 10 mm. and 20 mm. 

 from the umbonal apex, the very blunt ridge demarcating the area 

 from the escutcheon is ornamented by minute, transversely elongated 

 nodes or short raised lines, each measuring about one millimetre in 

 length. At a distance 'of about 10 mm. below the umbo the ribs on 

 the flank become bent down in the middle in angular form and 

 become more widely spaced. The transition from the straight to 

 the angularly bent flank-ribs is somewhat sudden. Three or four 

 bent ribs are successively produced, having a more delicate anterior 

 and more robust posterior limb, and an increasing acuteness of the 

 angle. Subsequently, an anterior and a posterior series of inclined 

 ribs may be spoken of, not strictly coinciding with each other in 

 number. The anterior ribs, numbering about 14-16 in the adult, 

 often cease to extend to the frontal margin after the shell has attained 

 half its adult dimensions ; their upper, anterior terminations are 

 situated on the flank at an increasing distance from the margin, and 

 forming acute angles with these, with upwardly directed apices, are 

 several short, steeply inclined, weak and rather broader ribs. Each 

 of these is only a few millimetres in length and might be considered 

 to represent the downwardly bent anterior termination of a rib of 

 the anterior series. The most forward portion of the flank is usually 

 devoid of sculpture and marked only by ridges and furrows of 

 accretion. The lowest ribs of the anterior series become in 

 varying degree irregularly nodose or broken into wavy lines of nodes 

 with loss of regularity near the lower valve margin. The ribs of the 

 posterior series have their upper termination at first close to the 

 carinal fold, and successively at a gradually increasing distance 

 from this, so that a narrow portion of the flank adjacent to the fold 

 is smooth. These ribs are not developed to the posterior end of the 

 flank in the adult, but there is here a smooth tract only marked by 

 lines of growth. The ribs of the posterior series, developed to the 

 number of about 14, are very steeply inclined, and all have a slight 

 forward slope. They are of rounded form, are about 3 mm. in 

 breadth, and have slightly narrower interspaces. 



The area is relatively narrow, and is marked throughout the adult 

 period by a longitudinal linear depression. In the posterior half of 

 the area this marks off a narrow superior and broader inferior 

 portion. There is no defined inner carina, and the line of delicate 

 and imperfectly developed nodes which marks the inner limit of the 

 area at a distance of 10-20 mm. from the umbonal apex is a 

 transitory feature, and does not persist with subsequent growth. 

 The escutcheon is relatively very long and is of lanceolate form, and 



