fi-om the Shales of the Northumherland Goal-field. 81 



similar are they, indeed, to those of the small species (0. ele- 

 gans) that it is impossible to deny the high probability of their 

 having 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 sj)ecimens of this large oper- 

 culum have occui-red. 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 opei'culum is two and a 

 half inches in diameter. It has a finely granulated surface, 

 and the contoiu- 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 and 

 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. ohliquus has been found connected with the 

 palatal teeth and scales. This and the scales will presently be 

 described along with the oral armature of that species. 



Now, should it be denied that these large opercular plates 

 belong to the Ctenodi^ it may be asked to what other fishes of 

 the coal-measures can they be assigned? Rhizodus, Rhom- 

 hoptychius^ 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- 

 hoptychius 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- 

 bability 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 tliis 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 Ctenodi, which 

 have been already estimated at four or five feet long. These 



