and Systematic Position o/* Cheirolepis. 245 



being placed near the angle of divergence. This plate is 

 marked x in Pander's figures, and seems to coiTespond to a 

 similar though somewhat smaller one seen in most Palaio- 

 niscida, and which in Quenstedt's previously quoted figure 

 of the head of Fahroniscus islehiensis is marked as ^^prcroper- 

 culumy IIow tar it represents a praoperculum is doubtful, 

 thougli it certainly does occupy a very analogous position to 

 that of the great pra^opercular cheek-plate in Folypterus. 

 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 hyomandihxdar 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, is 

 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 articidar and ancjidar elements (fig. 7, ag) 

 are recognizable ; but I have never succeeded in detecting 

 any inner or spJenial plate, though I have often seen it in 

 many Carboniferous Pala^oniscidte. 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 Xos. 255 and 435 of the Hugh-^Iiller collection ; and 

 though Pander states that he was unable to detect it, yet the 

 plate marked 48 in his tab. viii. fig. 2, as an element of the 

 shoulder-girdle, clearly corresponds with it both in form and 

 position. It is a naiTow, elongated, thin plate, with acute 

 anterior-superior and posterior-inferior angles, and placed 

 obliquely on the side of the head, betw^een the suspensorium 

 and the shoulder-girdle. The suhoperculum {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 hranchiostegal rays, described and figiu'ed by Agassiz, 

 were not observed by Pander nor by Prof. Huxley, though 

 he accepts and quotes Agassiz's statement regarding them. 



