The Invertebrate Fauna of the Uitenhage Series. 195 



dealt with, and whatever may ultimately become known regarding 

 their exact relationship to H. atherstoni, which they so closely 

 resemble, it is certain that so far as present evidence goes, this 

 relationship must be presumed to be very intimate. 



HOLCOSTEPHANUS WILMAN^B Sp. nov. 



Plate IX., figs. 1, la. 



Description of a Single Specimen. The shell is laterally much com- 

 pressed and is discoidal in form ; the flattened flanks of the ultimate 

 whorl are relatively extensive, while the peripheral area is narrow and 

 presents a somewhat flatly convex surface. The whorl is narrow and 

 highly arched in section, and the height of the whorl-section, 

 measured at the middle, is almost equal to the breadth, which is 

 greatest at the umbilical margin. The whorl, however, is almost 

 parallel-sided, so that the breadth of its section taken at the middle 

 of the flank is little less than at the umbilical tubercles. The in- 

 volution is such that about two-thirds of the preceding whorl are 

 embraced. 



The umbilicus is relatively shallow, and occupies one-third of the 

 total diameter. The umbilical rim is not abruptly defined, but the 

 surface of the flank curves down somewhat gradually to the umbilical 

 wall, which falls very steeply to the spiral suture. Primary umbilical 

 ribs commencing at the suture slope back very obliquely, and, quickly 

 gathering strength, swell up at the umbilical rim to form strong, 

 elongated, much-compressed tubercles, present to the number of 

 eighteen in the ultimate whorl. The secondary ribs proceed from the 

 tubercles chiefly in groups of three, but there are occasionally four ribs 

 in a group in the ultimate whorl, and frequently a rib is inserted so 

 as to terminate between two tubercles. The ribs at first swing 

 forwards from the tubercle, and bend back at the middle of the flank 

 to take an almost direct course across the periphery. The ribs are 

 prominent in character, and on the peripheral area in the anterior 

 part of the last whorl their crests are 4 mm. apart. 



Immediately behind the oral margin is a strong constriction, and a 

 portion of a lateral auricular extension is preserved. 



Dimensions. 



Greatest diameter 90 mm. 



Greatest breadth of the last whorl in cross-section 30 



Height of the last whorl at the centre, in section 28 



Greatest diameter of the umbilicus, measured from the 



umbilical rim between the tubercles 30 ,, 



