STRUCTURE OF CERTAIN PALEOZOIC DIPNOI. 193 
distance outwards from the middle line. Another result of the greater 
breadth of the tooth-plate is that it is supported posteriorly, notj as in 
Sagenodus, by a bracket on the inner face of the upper border of the angular, 
but by a pocket in the same position on the outer face (see fig. 27, E, p. 196). 
"The "splenial" is a much weaker bone than in either Sagenodus on Cerdtodus; 
its symphysial end is fairly strong and some part of it is commonly to be 
seen projecting in front of the mandibular teeth, but behind the symphysis 
it is so thin that it has usually been folded or crumpled beneath the tooth- 
plate in fossilization. 
The posterior edge of the " dentaries " is thickened and polished precisely 
as in Sagenodus, and no doubt there were gular plates fitting against it, but 
so far we have not found any bones that we could identify as gulars of 
'Ctenodus. 
Fig. 24. 
" Ctenodus intemiptus, Barkas." 
Pterygoid -with teeth and parasplienoid, oral aspect, X J. 
Oil shale of Broxburn, Midlothian (1902-73, R.S.M.). 
The left pterygoid is entirely, the right very nearly in natural 
articulation with the parasphenoid. 
The Operculum seems to have been exactly like that of Sagenodus. 
"Several specimens in the Atthey Collection are too large for any known 
?kull of Sagenodus, and these at least presumably belong to Ctenodus ;■ hni, 
apart from maximum size, we can point to no definite character by which 
opercula of the two genera may be distinguished. A sub-operculum was 
doubtless present, but we have not yet succeeded in identifying it. 
The Shoidder- Girdle. 
As already stated, we have never yet seen well-preserved bones of 
the shoulder-girdle associated with undoubted remains of Ctenodus. 
13* 
