204 Annals of the South African Museum. 



very closely related, I believe that the following points fully justify 

 the separation. In H. modderensis the whorl increases more rapidly 

 in breadth and the involution is stronger than in H. schenki. The 

 umbilicus is relatively narrower and the umbilical wall falls more 

 steeply in H. modderensis, in which also the umbilical ribs, at least 

 in the ultimate whorl, have a definite posterior inclination when 

 traced up towards the tubercles. In cross-section, the whorl is 

 considerably broader in proportion to height than in H. schenki. 

 Further, not only is the ribbing of H. modderensis at a com- 

 parable stage somewhat coarser, but the costae as they pass 

 from the tubercules across the periphery mostly follow a straight 

 course, while in H. schenki they have a well-marked forward 

 inclination. 



It is scarcely necessary to make a detailed comparison between 

 H. modderensis and H. rogersi, described above. H. modderensis 

 is a much more inflated and globose shell, and the rate at which the 

 whorls increase in breadth is considerably more rapid ; the umbilicus 

 is also relatively narrower, and the breadth of the whorl- section 

 proportionately greater than in H. rogersi. 



H, baini (Sharpe) * is no doubt a related shell, but H. modder- 

 ensis is well distinguished from this by its inflated form, the great 

 breadth and diminished height of the whorl-section, the rapid 

 increase in breadth of the whorl, the narrower umbilicus, and the 

 direct course of the secondary ribs as they cross the periphery. 

 H. modderensis is also considerably more densely ribbed than 

 H. baini. 



One of the forms figured by Pictet as Ammonites bidicho- 

 tomus Leym.,t which Pavlow considered identical with Oppel's 

 H. schenki, appears to be well distinguished from H. schenki by 

 the coarser ribbing, the direct course of the secondary ribs, and 

 the absence of noticeable constrictions. As far as it is possible 

 to make comparison by means of Pictet' s figures, the Swiss shell 

 seems to approach more closely to H. modderensis, though at 

 the stage represented it is less strongly involute. A satisfactory 

 comparison between this figure of a small specimen and the 

 relatively large specimen of H. modderensis is, however, not 

 possible. 



A specimen from Neuchatel figured by Pictet as Ammonites astie- 

 rianus d'Orb.J was united with Ammonites spitiensis Blanford by 



* Sharpe (1), p. 197, pi. xxiii., fig 2. 



f Pictet and Campiche (1), pi. xli., fig. 3 (1860). 



I Pictet and Campiche (1), pi. xliii., fig. 2 (1860). 



