and Systematic Position of Cheirolepis. 245 
being placed near the angle of divergence. This plate is 
marked « in Pander’s figures, and seems to correspond to a 
similar though somewhat smaller one seen in most Palzo- 
niscide, and which in Quenstedt’s previously quoted figure 
of the head of Palwoniscus islebiensis is marked as “ preeoper- 
culum.”’ How far it represents a preoperculum is doubtful, 
though it certainly does occupy a very analogous position to 
that of the great preeopercular cheek-plate in Polypterus. 
Above the margin of the anterior limb of this plate is fre- 
quently seen another portion of bone (fig. 7, y), the interpre- 
tation of which does not seem very clear, but which may 
very possibly be a portion of the hyomandibular exposed from 
under the previously described plate. The lower jaw, long 
and powerful in accordance with the great backward extent 
of the gape, was undoubtedly the strongest of all the bony 
parts of the head, as its contour, like that of the clavicle, 1s 
easily recognizable in most specimens. Its dentary portion 
(figs. 1 & 7,d) has been well figured by Pander, and is pecu- 
liar in presenting on its lower margin a wide shallow notch 
rather in front of its middle, and immediately above which 
the centre of ossification was placed. Besides the dentary 
portion, distinct articular and angular elements (fig, 7, aq) 
are recognizable; but I have never succeeded in detecting 
any inner or splenial plate, though I have often seen it in 
many Carboniferous Paleoniscide. The operculum (fig. 7, op) 
seems to have been a very delicate plate, as it is only in very 
few specimens that any trace of it is seen. However, it is 
unmistakably shown in one of Lord Enniskillen’s specimens, 
and in Nos. 255 and 435 of the Hugh-Miller collection ; and 
though Pander states that he was unable to detect it, yet the 
plate marked 48 in his tab. vi. fig. 2, as an element of the 
shoulder-girdle, clearly corresponds with it both in form and 
position. it is a narrow, elongated, thin plate, with acute 
anterior-superior and posterior-inferior angles, and placed 
obliquely on the side of the head, between the suspensorium 
and the shoulder-girdle. The suboperculum (s.op) is also 
rarely shown, and I have come across no specimen in which 
the whole of its contour is distinctly exhibited; to judge, 
however, from its remains, it seems to have been a somewhat 
square-shaped plate, placed immediately below the inferior 
margin of the operculum. ‘This is undoubtedly the plate 
marked 3 in Mr. Powrie’s figures, and which he supposes 
““ may have represented the operculum.” 
The branchiostegal rays, described and figured by Agassiz, 
were not observed by Pander nor by Prof. Huxley, though 
he accepts and quotes Agassiz’s statement regarding them. 
