42 
Annals of the Transvaal Museum 
Measurements : 
A 
B 
C 
Diameter ... 
261 mm. 
(i-oo) 
323 mm. 
(i-oo) 
232 mm. 
(I-oo) 
Height of last whorl 
90 „ 
(0-35) 
no „ 
(0-34) 
72 „ 
(0-31) 
Thickness of last whorl . . . 
70 » 
(0-27) 
90 „ 
(0-28) 
54 
(0-23) 
Height of penult, whorl... 
48 „ 
(o-i8) 
60 ,, 
(0-19) 
34 M 
(0-15) 
Thickness of penult, whorl 
32 „ 
(0-12) 
46 
(0-14) 
— 
— 
Diameter of umbilicus . . . 
108 ,, 
(0-41) 
136 „ 
(0-42) 
108 ,, 
(0*47) 
A . Specimen with broadest second lateral saddle. 
B. Specimen with narrower second lateral saddle. 
C. Specimen with narrowest second lateral saddle. 
The outer portion of the lobe-line of this species shows a great resemblance 
with that of Acanthoceras, especially with that of the group of A. Deverianum 
d’Orb. The lobe-hne of A . Deveroide de Gross, as given by Kossmat (12, p. 125), 
corresponds remarkably well with that of the present species. Except for the 
presence of a row of tubercles in the place of a keel, the type of ornamentation 
agrees also fairly well. It is a pity that the complete lobe-line of the mentioned 
species is not known. The inner portion of the lobe-line of the present species 
differs considerably from that of A. Newhoidi Kossmat for example (12, 
PL XIV, fig. 2), especially through the presence of two dorsal saddles. 
Without doubt this species belongs to the genus Movtonicevas Meek. It 
corresponds very well with the diagnosis given by Pervinquiere ( 18 , p. 227). 
However, Pervinquiere states, that the first lateral lobe has practically the 
same length as the external lobe, and this does not agree at least with one of 
our specimens, in which the external lobe is much shorter than the first lateral 
lobe, as in Acanthoceras ornatissimum Stol. Pervinquiere says, that the 
first lateral lobe is bifid or at least ends in two equal points. De Grossouvre 
(10, p. 66) goes further and states, that the end of the first lateral lobe is 
rounded and that its secondary lobes are sub-equal, that is to say, the charac- 
teristic terminal fork of the lobe in Acanthoceras and Stoliczkaia is not present 
or, at the most, only indicated. A comparison with the lobes figured of our 
specimens shows, that they do not agree with this description, on the contrary, 
they agree in this respect remarkably well with the lobe-line of the genus 
Peroniceras de Grossouvre (10, p. 93). Pervinquiere has pointed out, that 
the beautiful forking of the first lateral lobe of Peroniceras occurs also in 
Gauthiericera.s de Grossouvre ( 18 , p, 249). Our specimens now prove, that 
it is neither a distinctive character between Peroniceras and Mortoniceras. 
The only other character by which Peroniceras can be distinguished from 
Mortoniceras is the possession of three Jreels instead of one. However, Kossmat 
has shown (12, p. 88), that Mortoniceras Stangeri Daily sp. is a typical Peroni- 
ceras de Grossouvre in its young stage, and Pervinquiere mentions the fact 
( 18 , p. 248), that in the young stage there is but one keel in the genus 
Peroniceras and that each of the lateral keels may be replaced by a row of 
tubercles in the adult. The possession of three keels instead of one cannot, 
apparently, be considered to be a character, which is of generic value, at least 
in this case. There are, therefore, no characters which distinguish the genus 
Peroniceras from Mortoniceras and the first has, therefore, to be regarded as 
a synonym of the last. 
Another question is whether the relation between Acanthoceras and 
Mortoniceras is not closer than generally accepted. The most weighty character, 
separating these two genera into different families is the possession of a keel. 
Mention has been made above of the group of Acanthoceras Deverianum which 
has a row of tubercles instead of a keel. After Peroniceras has shown us how 
