200 Annals of the South African Museum. 



time, the slope of the umbilical wall of the last whorl becomes 

 slightly less steep. 



There are sixteen strong umbilical marginal tubercles in the last 

 whorl preserved, and from these the ribs of the flank and periphery 

 proceed most frequently in groups of three, though occasionally only 

 two ribs are given off from the nodes. Sometimes a rib may be 

 situated between two nodes at one of its extremities, while springing 

 from a node at its opposite end. These relations of the ribs, how- 

 ever, are the same in H. baini. The ribs as they depart from the 

 tubercles to cross over the periphery are not forwardly inclined, but 

 pass almost straight across the whorl. In the most advanced 

 portion of the specimen the crests of the ribs upon the peripheral 

 area are situated at a distance of 6 mm. apart. Well-marked 

 constrictions are present as in H. baini. 



Dimensions. 



Greatest diameter of specimen 92 mm . 



Greatest breadth of last whorl in cross-section 53 , , 



Greatest height of last whorl at the centre (section) 24 ,, 



Greatest diameter of the umbilicus, measured from the 



umbilical rim between the tubercles 45 ,, 



Occurrence. This specimen is believed to come from the Sunday's 

 Eiver. 



Remarks. There can be no douht that the form here represented 

 is very closely related to H. baini, and I was for some time 

 undecided whether definitely to identify the two. Allowing for the 

 difference in dimensions between this specimen and the examples of 

 H. baini that I have seen, which are smaller, the only apparently 

 valid character of distinction is in the disposition of the ribs. In 

 H. baini the ribs proceeding over the whorl from the nodes have a 

 well-marked forward throw, but such an inclination is not seen in 

 the specimen above described. Mr. G. C. Crick, who has kindly 

 examined the specimen, considers that this is a significant point of 

 distinction, but admits that in other respects the two forms are very 

 closely similar. With regard to the inclination of the ribs as a 

 constant and reliable character, it may be remarked that in the 

 British Museum collection a specimen of H. baini (registered 52052) 

 which strikingly resembles Sharpe's type in all other points, agrees 

 with it also in showing the same inclination of the ribs. It therefore 

 appears evident that in this character of the ribbing we have a 

 distinctive feature for separation, whatever this alone may be worth. 

 There is unfortunately as yet only a single specimen available for 

 comparison, and since in other respects it shows such close agree- 



