STRUCTURE OF CERTAIN PALiEOZOIC DIPNOI. 
191 
for they very rarely occur as isolated bones, whereas isolated roof-bones of 
Sagenodus are abundant. 
The circumorbitals of Ctenodus are shown only in one specimen amono- 
our material, and this is represented in outline in fig. 23, B, p. 192. In 
number and in general arrangement they appear to differ little from the 
circumorbitals of Sagenodus, except that a small- bone, partially embraced by 
the squamosal and post-frontal, occurs between them and the bones which 
actually enter into the orbit. This, however, may be an arrangement 
peculiar to " Ctenodus interruptus." 
In addition to the circumorbitals there are, as in Sagenodus, some bones 
filling in the space to the outer side of the nasals. They are not completely 
shown in any specimen that we have seen, but they obviously vary a good 
deal in shape (mar., fig. 21, p. 189, and fig. 23, p. 192). The single one shown 
in fig. 21 is represented as an impression of the underside, which is all that 
had been seen of it when the figure was drawn. 
Fm. 22. 
Fragments of the prenasal fan of Ctenodus, x -J. 
The Palate. 
A comparison of fig. 10, p. 176, and fig. 26, p. 195, will show the close 
resemblance between the bones of the palate in Ctenodus and Sagenodus. 
The tooth-plate of Ctenodus is larger (fig. 8, p. 173), the pterygoid behind it 
is of rather more slender form, and the parasphenoid (fig. 25, p. 194) has 
a more prominent median ridge on the buccal face of the lozenge. The 
shaft of the parasphenoid is more abruptly expanded, and shows a pair of 
pits close behind the apex of the lozenge. On the cranial surface the ridges 
and grooves of the shaft are much more numerous than in Sagenodus (cf. 
fig. 9, C, p. 174), and are continued forward to the centre of the lozenge, 
where they nearly meet the corresponding ridges from the anterior process. 
In the larger parasphenoids, some of which are nearly a foot in length, 
the median ridge of the lozenge is swollen and club-shaped (fig. 26, p. 195), 
and it seems to be usual for the right-hand pit of the pair behind it to com- 
municate directly with the central groove of the shaft, as shown in fig. 25, C, 
p. 194. The appearance is suggestive of the crossing of a pair of longitudinal 
LINN. JOUBN. — ZOOLOGY, VOL. XXXV. 13 
