STEUCTURE OF CERTAIN PALEOZOIC DIPNOI. 
181 
in outline, but the form shown in fig. 20, p. 186, is a good average. The 
variation consists for the most part in the greater or less development of the 
hinder lobe of the bone. There is every gradation^ independently of size, 
from a form in which the body of the bone is fairly symmetrically disposed 
under the centre of the hinge-line, to an extreme but quite common pattern 
in which the hinder border is so prominent that the centre of the bone lies 
below tlie posterior knob, behind the hinge-line altogether. In this latter 
pattern of operculum the posterior knob is -always greatly developed iind 
the anterior one almost obsolete. Opercula of this type have been figured 
by Miall (1881, fig. 7, where they are shown upside down), and by Williston 
(1899, pis. 35 & 36) in his description of S. copeayius. 
A small bone, hitherto undescribed, which we take to be the Sitb- 
opercidum, occurs in sever;il of the crushed heads of Sagenodus which we 
Fig. 16. 
A y- 
Sagenodus. 
.A, left? g'ular, inner surface; B, diagram, showing the 
prCibable position of the gulars. 
have examined ; it is shown, for example, on the slab, part of which is 
represented in fig. 18, p. 183, and in the head in the Manchester Museum 
(L. 10904) described and figured by Watson and Day (1916). It is a 
bone of an inch to an inch-and-a-half in length in an average skull, and is 
recognizable by its triangular point (see fig. 18, A, p. 183). Completely 
preserved examples, such as those represented in fig. 7, p. 172, show that 
half the bone was fairly thick and strengthened by a rounded ridge, while 
the other half consisted of a thin flange which might easily be lost. Its 
disappearance would give the rest of the bone a much more pointed shape, 
such as is seen in fig. 18, p. 183. The sub-operculum of Ceratodus (inter- 
operculum, Huxley), to judge by a dried skull, would much resemble this 
bone if its cartilaginous fringes were ossified. No specimen that we have 
seen shows this bone in its natural position, but on two out of the three slabs 
on which we have found it, it lies close to the squamosal, which may indicate 
that it was applied to the front rather than to the hind border of the 
