On the Senonian Ammonite Fauna of Pondoland. 
139 
represented by only one example. The writer is of opinion that the common 
occurrence of these types of variation in Ammonites (not mutations, since, 
apparently, they are contemporaneous) may be taken to indicate a form of 
" dimorphism," or rather of " polymorphism." The " polymorphism," 
however, necessitates agreement in the other characters, which would have 
to be ascertained to exist in a larger number of specimens than are available 
in the present instance. Moreover, it is difficult to stipulate what would 
be agreement in, e.g., the suture-lines ; and since the change of a species- 
group into another and later one occurs by means of adaptive radiation, 
graphically represented by fans or a multitude of more or less radiating 
lines rather than by a single line in the customary genealogical tree, 
opinions would differ greatly as to the "agreement" in a given species-group. 
The fact that the adult whorls of the various forms here discussed become 
similar, but not " catagenetic," shows that this iC polymorphism " is a 
phenomenon quite different from the simplification of the ornament on the 
outer whorls of many Ammonites, which may tend to perfection of adapta- 
tion in, e.g., Oxynoticeras, and which often, like the approximation of the last 
few septa, marks only the slackening of growth on reaching maturity, but 
which, also, may be simply correlated with the overlap of the mantle in 
the mature stage. 
Genus Pseudoschloenbachia, Spath. 
37. Pseudoschloenbachia umbulazi, Baily sp. 
(Plate VI, fig. 5.) 
1921. Pseudoschloenbachia umbulazi (Baily), Spath, Zululand, p. 240, 
pi. xx, fig. 2, text-fig. B, p. 241. 
1921. Schloenbachia umbulazi (Baily), v. Hoepen, loc. cit., p. 35, pi. viii, 
figs. 6-9, pi. ix, figs. 1 and 2. 
1921. Pseudoschloenbachia umbulazi (Baily), Spath, Pondoland, table 
to p. 50. 
The measurements of typical examples agree with those given by 
Dr. van Hoepen, e.g. No. C 19427 has dimensions 50 — 52 — 28 — 14. The 
inner whorls of this example were figured by the writer {loc. cit., text-fig. 
Bl, p. 241). They show that at a diameter of 32 mm. the thickness = 30 
per cent., and the umbilicus = 18 per cent., so that the whorls become 
relatively thinner with age, whereas the umbilicus narrows. At a diameter 
of 21 mm. the thickness has increased to 33 per cent. In the var. acuta, at 
the same diameter, the whorl-thickness is only 26 per cent. 
The writer has pointed out that these Pondoland examples represent 
more strongly ornamented varieties, with increasing tuberculation round 
