from the Shales of the Northumberland Coal-field. 81 
similar are they, indeed, to those of the small species (C. ede- 
ans) that it is impossible to deny the high probability of their 
faving belonged to species closely allied to it; and at the 
same time they are readily divisible into species. 
The largest of these plates is five and a half inches in dia- 
meter; it is almost orbicular, with the hinge-line flattened, 
and with a rounded process projecting a little at each end 
limiting its extent: three or four specimens of this large oper- 
culum . occurred. The next in size, of which I have two 
specimens, is four inches across its longest diameter ; it is 
considerably elongated in the transverse direction, being pretty 
regularly oval; the hinge-line is straight, and strongly defined 
by two lateral processes. ‘The third operculum is two and a 
half inches in diameter. It has a finely granulated surface, 
and the contour is circular and somewhat sinuous; the hinge- 
line is well defined by two rather acute processes, and is longer 
than usual. Another orbicular species, about the same size 
has the margin less sinuous, and the hinge-line shorter an 
not so well marked by lateral processes. Besides the above, 
one or two much smaller but rather obscure kinds have oc- 
curred, as well as separate plates of C. elegans. And one be- 
longing to C. obliquus has been found connected with the 
ae teeth and scales. This and the scales will presently be 
escribed along with the oral armature of that species. 
Now, should it be denied that these large opercular plates 
belong to the Ctenodt, it may be asked to what other ri oe of 
the coal-measures can they be assigned? Rhizodus, Rhom~- 
boptychius, and Megalichthys are the only large species that 
occur to which they could, according to our present knowledge, 
belong. The other large coal-measure fishes are all Placoids, 
and are therefore out of the question. Rhizodus and Rhom- 
boptychius are, however, apparently closely allied to Holopty- 
chius, in which the operculum is composed of more than one 
piece and in the former therefore the gill-cover is in all pro- 
ability similarly composed; while in Megalichthys the cha- 
racter of the enamel and form of the operculum sufficiently 
_ prove that the large opercula above alluded to cannot belong 
= to it. 
In the same shales likewise occur strong well-arched ribs, 
_ the largest measuring six or eight inches in length. ‘These 
_ also probably belong to the larger Ctenodi; and this probabi- 
lity becomes almost confirmed when we refer to the fact of the 
_ ossification of the ribs in C. elegans, and that the proportion 
_ they bear in this species to the length of the fish is just about 
_ the same that those large ribs bear to the larger Ctenod?, which 
have been already estimated at four or five feet long. These 
