ERIK A: SON STENSIO 
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separated from each other by a horizontal sinus penetrating from the back. The sinus is most 
strongly developed towards the posterior part of the caudal region, as is seen intextfig.67 A. 
The elements Iv are not in contact with each other but are separated by fairly 
large gaps, through which the intersegmental vessels may have had their exit from the 
haemal canal. As far as can be discovered each element Iv is situated opposite an inter¬ 
dorsal and each element Bv opposite a basidorsal. From this one might conclude that 
the element Iv represented an interventral and the element Bv a basiventral, but the 
conditions are probably more complicated than this. For it seems to me probable that 
the element Iv in each segment also includes a proximal part of the basiventral, while 
the element Bv would, if this were correct, represent only the distal part of a basiventral, 
i. e. it would represent only a haemal spine. This view is, at least to a certain excent, 
supported by the relations stated above between the elements Bv and the elements Iv 
in the anterior part of the caudal region. 
With regard to this last-mentioned element {Iv) it ought finally to be remarked 
that I was unable to ascertain whether it was fused with that of the opposite side. In 
any case, however, it has formed with this a semicircle which has surrounded the ventral 
half of the chorda in each segment. We thus have the beginning of the formation of a 
vertebral body, a hypocentrum, but the chorda was certainly very little affected by this. 
As will be seen from what has been mentioned, the vertebral column in B. mougeoti 
has retained its original segmentation as in the sturgeons. It is however somewhat more 
specialized than in these in the following respects: 1) a stronger degree of ossification; 
2) indications of the formation of vertebral centra; 3 ) the beginning of reduction in the 
contact between the basidorsals and the chorda; 4) the assumed fusion between the 
interventral and the proximal part of the basiventral in each segment. In these respects 
Birgeria resembles most closely Amici and certain of the higher fossil Ganoids. 
The skeleton of the unpaired fins. 
The anal fin (A, PI. 21, fig. 4) is only preserved in specimen P. 155. It was 
apparently well developed and had in this specimen at the base a length of at least 
20 cm. The lepidotrichia were very numerous; their total number was probably about 70. 
Their distal parts are not preserved, and the shape of the fin cannot therefore be seen 
from the material present. They are, as is usual in Palaeoniscids, situated close together, 
but while in Palaeoniscids in general they are closely jointed throughout their length, in 
B. mougeoti they have here a short unjointed proximal part, which is also found in the 
dorsal and caudal fins. Birgeria mougeoti thus shows in this respect a certain specialization 
in the direction of the higher ganoids. The lepidotrichia of the anal fin were, however, 
as usual, far more numerous than the endoskeletal radials, and /it is also noteworthy that 
they had no ganoine covering. 
The endoskeleton of the anal fin may have consisted of about 25 radials, 22 of 
which are partly preserved. The anterior 16 (Ps -f- Ms, PI. 21, fig. 4) are each practically 
occupied throughout their length by a single ossification, extending from the distal ends 
of the haemal spines to the basal ends of the lepidotrichia, while the remaining 5 have 
two ossifications, the proximal one (Ps, PI. 21, fig. 4), which reaches close to the distal 
ends of the haemal spines, being the longest. 
