226 
ERIK A : SON STENSIO 
The lower jaw is represented only by two fragments of bone. One of them is a 
part of the articular {Art, text fig. 72; PI. 3 1, figs. 1, 3 ) or of the articular component if 
a large and long bone of the same sort as in Boreosomus arcticus was developed in the 
Meckelian cartilage. The other fragment is a dorsal part of the mixicoronoid ( Mco , text 
fig. 72; PL 3 1, figs. 1, 3 ; PI. 32 , fig. 5). As far as can be seen, no coronoid process or 
indications of one are present, the labial margin of the mandible being straight right 
to the posterior end. The articulatory fossa on the articular (or possibly the articular 
component) is clearly visible in profile (PI. 3 i, fig. 1). 
The mouth is large and extended far back as in the majority of Palaeoniscids. 
The hyomandibular {Hm, text fig. 72; PI. 3 i, figs. 1, 3 ), which is situated immediately 
behind the palatoquadrate, occupies its usual oblique position, i. e. it is directed backwards 
and downwards. It is a large bone, bent somewhat above the middle in such a way 
that the anterior border is concave and the posterior one, on the whole, convex. The 
centre of ossification lies just at the bend, and on the posterior border opposite this 
centre issues the robust processus opercularis (pr. op). From the ossification centre 
the bone gets wider towards both ends, especially towards the dorsal one. Both the 
ends are in their present state abruptly truncated, and there was evidently a cartilaginous 
epiphyse on each of them. The processus opercularis has also probably been provided 
with a similar epiphyse. With regard to the signification of this process I refer to my 
account on pp. 2i3—514 above. No canal or foramen for truncus hyoideo-maridibularis 
facialis is present. — The bone substance is radiate as in the corresponding bone in 
other Palaeoniscids. 
Connected with the ventral end of the hyomandibular we find on specimen P. 174, 
both on the right and left sides, a small but rather thick spongy bone ( Sympl, text 
fig. 72; PI. 3 i, figs. i, 3 ) which was probably surrounded by cartilage. The bone is 
situated immediately behind the articulation between the quadrate and the lower jow 
and probably corresponds to a part of the symplectic, which, as we know, occupies a 
similar position i sturgeons, in which it may also sometimes be ossified (Parker 1873, 
p. 257, Pi. XX; van Wijhe 1882, p. 224; Gaupp 1904, p. 901). In addition Broom (1909a, 
p. 260) states that he observed an ossified symplectic in the so-called Oxygnathus broxvni 
(cf. p. 209 above). 
The conditions of the stylohyal are still uncertain. On the left side of specimen 
P. 174 there is certainly an impress of a badly preserved bone that might be the 
stylohyal (Sthy? text fig. 72; PI. 3 i, fig. 3 ), but on the other hand the possibility cannot 
be denied that it may really be an impress of a scale or something that has no 
connection at all with the hyoid arch. 
Medially and somewhat ventrally of each half of the mandible we find on specimen 
P. 174 a large hour-glass shaped ossification in the ceratohyal {Chy, text fig. 72; PI. 3 i, 
figs. 1, 3 ; PI. 32 , fig. 5). This ossification is truncated at both ends and probably forms 
entirely or at least preponderatingly a perichondral diaphyse. That an ossificatory process 
had also begun in the part of the ceratohyal situated behind this ossification is also seen 
from specimen P. 174, where we find an impress of a thin bone lamella [ohy, text fig. 72: 
PI. 3 1, figs. 1, 3 ; PI. 32 , fig. 5) behind and adjacent to the ceratohyod ossification. Here 
we evidently have the beginning of an ossification on the part of the ceratohyal that 
