206 Annals of the South African Museum. 



HOLCOSTEPHANUS UITENHAGENSIS Sp. nOV. 



Plate XI. 



1905. Olcostephanus bainii F. H. Hatch and G. S. Corstorphine 

 (partim). The Geology of South Africa, p. 243, fig. 65 

 (right-hand specimen only). 



Description of a Single Specimen. The specimen is preserved 

 chiefly in the form of a cast which in the greatest portion of the 

 ultimate whorl has retained, though somewhat imperfectly, the 

 impress of the external costate ornaments. The specimen is also 

 slightly distorted by pressure. 



The shell has a laterally compressed aspect, and the whorls are 

 relatively narrow and highly arched in section. The flanks are broad 

 and flattened in form, while the peripheral area presents a somewhat 

 narrowly convex surface. The involution is such that about two- 

 thirds of the flank of the preceding whorl are embraced, thus leaving 

 a considerable space free between the spiral suture and the umbilical 

 margin of the preceding whorl. The greatest breadth of the whorl 

 is at the umbilical margin, though the breadth at the middle of the 

 flank is little less than this until the anterior portion of the ultimate 

 whorl is reached, when the whorl is relatively rather more inflated. 



The umbilical margin is abrupt, and the umbilical wall falls 

 almost vertically until traced to the last half of the ultimate whorl, 

 where the wall slopes with slightly decreasing steepness. Umbilical 

 ribs are not developed in the last whorl. The umbilical marginal 

 tubercles, present to the number of fifteen in the last whorl, are not 

 strongly compressed in form, and they are well spaced. The ribbing 

 of the shell is relatively fine, and in the ultimate whorl three or four 

 costae proceed from each tubercle, in addition to which there are 

 some costae which terminate between the tubercles and occasionally 

 a rib may become intercalated on the flank. There is no evidence 

 that any bifurcation of the costae occurs on the flank. The ribs 

 as they pass towards the periphery are forwardly inclined, more 

 noticeably so in the anterior half of the ultimate whorl, where, on 

 the peripheral area, adjacent ribs have their crests 6 mm. apart. 

 The development of constrictions is very weak and inconspicuous. 



Dimensions. 



Greatest diameter 22 cm. 



Greatest breadth of the last whorl in cross-section 9 ,, 



Height of the last whorl at the centre, in section 6 ,, 



Greatest diameter of the umbilicus, measured from the 



umbilical rim between the tubercles 7'5 ,, 



