402 MESSRS. HANCOCK AND ATTHEY 



Another specimen, showing half of the bones of the occipital 

 region, including the parietals, has occurred. This may per- 

 haps belong to C. cristatus ; at all events it is a portion of the 

 skull of one of the larger species, for it is of considerable size. 

 And that it cannot belong to either of the above two species is 

 proved by the form of the median occipital, which differs greatly 

 from that of both. It is strongly pointed in front, and the 

 lateral posterior margins are remarkably sinuous ; the surface- 

 structure, too, is different. 



The bones that are determinable in the above fragmentary 

 skulls are the median and external occipitals, the anterior occi- 

 pitals, the parietals, and the lateral or "skin bones" according 

 to Pander, of which only the three posterior are present. The 

 arrangement of these bones is precisely similar to that given 

 in Pander's restored figure of the cranial shield of Dipterus 

 (tab. 3, fig. 1), the principal difference being that they vary 

 even less in size than those in the figure ; the median occipital 

 in C. tuberculdtus is scarcely, if at all, larger than the other 

 bones. And the surface of the whole, which is in a good state, 

 is devoid of ornamentation, save that it is minutely and irre- 

 gularly granular and punctate ; but the species vary a little in 

 these respects. 



In Mr. Atthey's collection of separate cranial bones there 

 are, besides specimens of the above, the median occipital of 

 three or four other species, all varying a little in contour and 

 in surface-character ; so that we have here additional corro- 

 borative evidence that our shales contain six or seven species of 

 Ctenodus. 



The opercula likewise resemble those of Dipterus ; they are 

 large, stout, slightly convex, irregularly circular plates, with 

 one side of the margin a little flattened, and slightly produced 

 at each end of the flattened space ; the surface is punctate and 

 granular like the cranial bones. We possess six or seven 

 different forms of these gill-covers, two of which have been 

 identified as belonging to C. elegans and C. obliquus respectively. 

 But, for a particular description of the various opercula, we 

 must refer to our paper previously quoted, merely observing 



