2 X 4 
ERIK A : SON STENSIO 
would have, been developed, and the movements of the opercular apparatus would have 
proceeded in the same way as in the recent sturgeons, i. e. by contraction of the mus- 
culus constrictor superficialis II, which on the whole would have been rather slightly 
specialized. 
It does not seem impossible, however, that Traquair has gone a little too far in 
this. It certainly seems to be settled that there is no real processus opercularis present 
in the genera mentioned, but, according to what is shown by his description of the 
hyomandibular in Palaeoniscus (1877a, p. 17) and Cheirodus (1879, p. 366 ; PI. V, fig. 10), 
this bone has at its posterior margin a more or less marked corner, which might be 
imagined to be a process of this sort in a primitive stage of development. In Palaeoniscus 
this corner is so situated that, according to Traquair’s statement, it would correspond 
to the boundary between the operculum and the suboperculum. In Cheirodus, on the 
other hand, it is found farther dorsally, so tat it corresponds to the upper part of the 
medial surface of the operculum. I have myself observed in Gyrolepis too a corner, 
situated about as in Cheirodus. 
Whatever may be the case about this corner, it is obvious that in the Palaeoniscids 
we find both forms in which the musculature of the operculum has been comparatively 
primitive and forms in which it was rather highly specialized in the direction of the 
higher Ganoids an Teleosts. 
It may be added with regard to the hyomandibular in Boreosomus arcticus that it was 
probably not pierced by the truncus hyoideomandibularis facialis or any of its branches. 1 ) 
Of the opercular skeleton in Boreosomus arcticus nothing is present except what 
has been described and figured by Woodward (1912, pp. 292—293, PI. XIV, fig. 2). 
The skeleton of the unpaired fins. 
Of the unpaired fins there are only some lepidotrichia, probably belonging to the 
dorsal fin, which all are densely jointed throughout their length and divided dichotomically 
in their distal parts. 
The skeleton of the paired fins. 
I have nothing to add to Woodward’s description of the membrane bones of the 
shoulder girdle. 
The pectoral fins were fairly small and their lepidotrichia, at least in the Upsala 
specimen P. i 32 , were jointed only in their distal third (orig. spec, of Woodward’s de¬ 
scription 1912, and his PI. XIV, fig. 2). The endoskeletal radials of these fins were, at 
least partly, rather long. 
The ventral fins were probably small. Their lepidotrichia, a few of which are present, 
are fine and probably jointed throughout their length. 
: ) It is of interest to note that in the hyomandibular of Gyrolepis alberti (specimen belonging to the 
Palaeontological Institute of Upsala) I have found a rather coarse canal which was probably pierced by the truncus 
hyoideomandibularis facialis. The canal, which is situated near the posterior margin of the hyomandibular, passes off 
in a ventro-lateral direction and has its exit immediately antero-ventrally of the corner of the posterior margin. 
