188 Annals of the South African Museum. 



Remarks. Some characteristic points in H. atherstoni are the 

 strong, backwardly directed umbilical ribs, the steep umbilical wall, 

 the relatively narrow and profound umbilical cavity, and the con- 

 siderable breadth and inflation of the whorls. The involution until 

 the shell has attained large dimensions is such that the umbilical 

 wall of a given whorl falls upon the umbilical tubercles of the 

 previous whorl, wholly concealing the secondary ribbing. In the 

 large type-specimen of H. atherstoni, measuring 140 mm. in greatest 

 diameter, the last-formed whorl is beginning to show a little less 

 involution, and its most advanced part leaves the tubercles of the 

 previous whorl wholly exposed, and also the commencement of the 

 rib-bundles of the flank. 



It has been stated by several authors that H. atherstoni occurs in 

 the Valanginian and Hauterivian of Europe, and since such an 

 occurrence would be of high interest, both from a stratigraphical 

 and palseontological point of view, I have taken some pains to test 

 the value of these identifications. The result has been to convince 

 me that none of the determinations can be upheld with certainty. 



Pavlow considered Holcostephanus psilostomus Neum. and Uhlig,* 

 from the Neocomian of North Germany, to be identical with 

 H. atherstoni, or at least that the two forms represent no more than 

 varieties of a single species. Uhlig, on the other hand, believes this 

 view to be erroneous,! and points out that H. atherstoni has an 

 almost semicircular cross-section and greater breadth. There can 

 be no doubt that, though closely comparable, these types are not 

 identical. Another allied North German form, from the zone of 

 Horjlites radiatus (Lower Hauterivian) has been described and 

 figured by von Koenen J under the name Astieria aff. psilostoma. 

 In this, the rate of increase in the breadth of the whorls is not so 

 rapid as in H. atherstoni, and the ribbing is a little coarser. The 

 pronounced development of constrictions is another distinguishing 

 feature. 



Another European form which bears considerable resemblance to 

 H. atherstoni has been described by von Koenen as Astieria convoluta 

 from the Lower Hauterivian of North Germany. This appears to 

 have similarly inflated whorls and a narrow umbilicus, but the 

 number of umbilical tubercles is fewer than in H. atherstoni at a 

 comparable stage of growth, and the ribs are coarser and less 

 numerous. 



4 Neumayr and Uhlig (1), p. 149, Taf. xxxii., fig. 2. 



t Uhlig (4), p. 132. * von Koenen (2), p. 151, Taf. liv., fig. 2. 



von Koenen (2), p. 146, Taf. xxxix., figs. 4a, 46. 



