58 MESSRS. HANCOCK AND ATTHEY ON 



three at least of the larger species cannot be less than four or 

 five feet from head to tail. The sphenoid bones previously men- 

 tioned of C. elegans are together about half-an-inch long ; the 

 same bones of the large species alluded to must have been at 

 least eleven inches in length. 



From the fragments of a large sphenoid and palatal bones in 

 our possession, it is possible to restore this portion of a skull that 

 probably belonged to C. cristatus, which is not the largest spe- 

 cies ; and we are thus enabled to form an approximate estimate 

 of the size of the head, which must have been upwards of twelve 

 inches long and nine broad. Therefore, assuming the propor- 

 tions to be similar to those of C. elegans, we arrive at the con- 

 clusion that these large Ctenodi were not less than four or five 

 feet in length. 



This estimate of the size of these animals receives some sup- 

 port from the fact that very large opercular plates occur in the 

 same shales in which the palatal armature is found. - We have 

 in our possession six or seven diff'erent kinds of opercula, all 

 having the essential characters of those found in connexion with 

 the head of C. elegans ; that is, they agree with them in the 

 roundness of their form, in being composed of a single solid 

 piece, and in the surface-granulation and enamel. So similar 

 are they, indeed, to those of the small species {C. elegans) that 

 it is impossible to deny the high probability of their having be- 

 longed to species closely allied to it ; and at the same time they 

 are readily divisible into species. 



The largest of these plates is 5^ inches in diameter ; it is al- 

 most orbicular, with the hinge-line flattened, and with a rounded 

 process projecting a little at each end limiting its extent : thi'ee 

 or four specimens of this large operculum have occurred. The 

 next in size, of which we 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 '2.^ inches in diameter. It has a finely gra- 

 nulated surface, and the contour is circular and somewhat sinu- 

 ous ; the hinge-line is well defined by two rather acute processes, 



