282 Annals of the South African Museum. 



there are three secondaries to each primary rib. :;: On the outer whorl 

 the ornament, is considerably weakened. The umbilical border is 

 gently rounded and the slope slightly concave, except on the innermost- 

 whorls. The keel is very prominent and narrowed at its base, as in 

 the form of Pseudopliacoceras described by Ooster and in many 

 Dipoloceras (see ante, pp. 277 and 280). 



The smaller example, which has portions of the outer whorl 

 preserved (not shown in fig. 1 c of PI. XXV), has its last suture-line 

 at a diameter of about 115 mm., when, here also, excentrumbilicatioii 

 sets in. Unfortunately only the umbilical portion of the outer whorl 

 here is preserved. The (last) suture-line of the larger example, taken 

 at a diameter of 160 mm., apparently differs from that of the smaller 

 specimen, the latter (in fig. 1 c) being nearly half a whorl away from 

 the beginning of the body-chamber, at a diameter of 80 mm. But 

 though comparison at the same (relative) size is impossible without 

 breaking up the type, the dissimilarity, in the writer's opinion, is 

 accounted for by the fairly frequently observed simplification of the 

 last few suture-lines, in Ammonites generally, often accompanied by 

 equalisation of the elements. This is a phenomenon of individual 

 growth and cannot be applied to phylogeuy ; and it seems to the 

 writer that the suture-line of the smaller specimen (fig. 1 c) indicates 

 that descendants of this group would show the Sphenodiscus type of 

 suture-line, with adventitious elements. 



The suture-line of the present species differs from that of the less 

 compressed P. multiftdum, Steiumann sp.,t its nearest relative, chiefly 

 in the greater depth of the ventral lobe and the presence of a larger 

 number of auxiliaries (corresponding to its greater involution) a 

 character that also is not shown in the more evolute form figured by 

 Lasswitz.+ The suture-line of d'Orbiguv's A. roissyanus has this 

 deep ventral lobe, but differs in the auxiliaries. On the other hand, 

 in a specimen of P. aff. roissyaniim (from Escragnolles, in the British 



* This type of ornament is somewhat reminiscent of that of yeoharpoceras 

 [gen. nov.] cf. hugardianuni (d'Orbigny) in Pictet (/<. df., Pictet & Roux, 

 1847, pi. x, fig. 3), but the suture-lines are very different in the two stocks. 

 (Genotype = A. hugardianus, d'Orbigny , ' Pal. Fran*;. Ter. Cret.,' pi. Ixxxvi, 

 figs. 1 and 2.) 



f "Ub. Tithon and Kreide i. d. Peruan. And.," 'N. Jb. f. Min., etc.,' II, 

 1S81, p. 139, pi. vii, fig. 1 \_Schloenbachia acuto-carinata (Shum. sp.) Marcou]. 



I " Kreide-Amm. v. Texas," ' Geol. and Pal. Abh.,' N.F., vol. vi, Heft 4, 1904, 

 p. 22, pi. v, fig. 2. Schlagintweit (see below) questions Lasswitz's identifications, 

 but the present examples indicate that in the case of the secondaries it occa- 

 sionally is impossible to say whether they result from dichotomous branching 

 or are simply intercalated. 



Loc. cit. (1840), p. 302, pi. Ixxxix, figs. 1-3. 



