ERIK A : SON STENSIO 
have postponed these till below (Part II) in connection with the description of the 
Saurichthyids, in which the skeleton of the ventral fins is more completely preserved 
than in B. mougeoti. 
As far as I can see the pelvis in different Elasmobranchs is not equivalent. We 
must presumably assume that to the primitive pelvic plate, as it appears e. g. in 
Cladodontids and Pleuracanthids, there was added during the phylogenitic development 
more or less of the proximal parts of the radials. To the primitive pelvic plate — called 
by various authors a basal — a secondary component would thus have been added. 
In other words, in the fishes in which this process took place we can discern in the 
pelvic plate a primary component representing the primitive pelvic plate and a secondary 
component, arisen from proximal segments of the endoskeletal radials. 
A process of the same sort as this in the Elasmobranchs seems, to judge from the 
conditions in Saurichthys (cf. Part. II), to have taken place in primitive fossil Actinopterygii 
and even to have proceeded rather far. At the same time as this has happened the 
primary pelvic component has been more or less exposed to reduction. The metapterygoid 
elements seem entirely or partly to have fused with one another and with the secondary 
pelvic component, at the same time as they lost their importance and decreased in 
size. When the process continued later, the conditions have finally appeared that we 
find in higher Ganoids and Teleosts. 
The skeleton of the ventral fins in B. mougeoti — to connect our description with 
this — has probably been developed in a somewhat similar way to that of Saurichthys, 
and the plate that I referred to preliminarily as the pelvis probably consists here too 
both of a primary and a secondary component, which is probably also the case with 
the corresponding formations in the sturgeons. It is impossible to decide what were the 
conditions of the metapterygeal elements in B. mougeoti — whether one or more of them 
were fused to the pelvic plate. It is noteworthy, however, that they form together a 
rather long row, and thus that they could not have been much reduced, a fact that 
gives the .fin skeleton as a whole, as we found, an Elasmochanchian-like character. 
The occurrence of a long metapterygeal axis in the paired fins in primitive 
Elasmobranchs and Actinopterygians and a more or less centrally situated axis in the 
same fins in Crossopterygians and Dipnoans now seems, as has been put forward from 
several quarters (Abel 1919, pp. ii 3 , 185; 1920, pp. 335 — 336 ; Stromer 1920, pp. 17, 18; 
Jaekel 1919a, pp. 27, 3 o; Simroth 1891, pp. 340—351), to indicate that the usual paired 
fish-fins are something secondary and have most probably originated from a trans¬ 
formation of supporting or crawling organs. This transformation is due according* to 
Abel (1919; 1920), to an adaptation from a benthonic to a nectonic mode of life. An 
additional argument in favour of the view mentioned here seems to be present in the 
curious pelvis of the Coccosteids, in which, as we know, this element was probably in 
connection with the vertebral column (Stromer 1920, p. 17). 
As far as the fin-fold theory is concerned in this connection, it cannot of course, 
so well founded as it undoubtedly is, be considered as being essentially disturbed 
by the facts that have so far arisen from the palaeontological side. We must still 
remain of the opinion that it affords the best explanation of the origin of the paired 
extremities. 
