84 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



e a 



r f 



Fig. 136 Yoldiella frigida (f), Y. robusta (r), & (separately) Y. 

 curta(c). 



Fig. 137 Yoldiella argentinensis (a), Y. externa (e) & (separately) 

 Yoldiella inconspicua inconspicua (i), Y. i. profundorum (p) & Y. 

 i. africana (a). 



s I ° 

 Fig. 138 Yoldiella lucida (1), Y. similirus (s) & Y. obesa incala (o). 



Fig. 139 Yoldiella bilanta (b), Y. hanna (h) & Y. capensis (c). 



Figs 136-147 Outline comparisons of species by overlay technique. Each outline is centred with reference to the junction of the centre of the 

 mid-length axis with the centre of the mid-height axis. Outlines are drawn to a similar length. Each shell outline comprises the margin in 

 right lateral view, hindgut loop and anterior and posterior adductor muscles. Anterior, posterior ventral and dorsal limits are indicated 

 parallel to the relevant shell axis as too is the apex of the umbo. Each limit is identified by the first letters of the species name. The figures 

 are in the order of the discussion on the evolution of form. (See text pp. 82-86). 





