420 MESSRS. HANCOCK AND ATTHEY 



open, exposing to view the internal structure, when the concen- 

 tric lines of growth and minute radiating striae are sharply denned 

 over the entire surface. The scale of Ctenodus is never seen 

 with the internal structure thus exposed ; at least we have never 

 seen the concentric lines of growth and radiating striae pass 

 beyond the border, the under surface being usually exposed to 

 view. This is well shown in our second species, the specimen 

 being preserved on one slab in relief, the cast of the underside 

 in intaglio on the other. This specimen, too, enables us to judge 

 of the thickness of the scale, as it is evident the entire substance 

 of it is present, and that it is not torn open by the splitting of 

 the shale. 



The rectangular outline of these scales we have just pointed 

 out as peculiar ; and in this respect these large scales agree with 

 those we previously described of C. elegans and C. obliquus, the 

 former being the smallest known species of the genus. And here 

 we must not overlook the similarity both in form and size of 

 these large Ctenodus-scales to those of the so-called Ceratodus 

 Forsteri, as figured and described by Dr. A. Giinther in his va- 

 luable memoir on this remarkable Australian fish, recently pub- 

 lished in the "Philosophical Transactions." This resemblance 

 is very striking in our second species, in which the sides are 

 nearly parallel, being a little arched outwards, much in the same 

 way as they are in the recent species. In both forms the scales 

 are of an extraordinary size : those of Ceratodus Forsteri are 

 two inches and three-eighths long, and one inch and six-eighths 

 broad ; the largest Ctenodus-scale measures two inches and a 

 half in length, and an inch and a half in breadth ; and that of 

 C. elegans, which is quite a small species, is remarkably large 

 for the size of the fish. 



We have shown on a previous occasion that the dental plates 

 of Ctenodus imbricatus are so similar to those of the Australian 

 fish that without other aid they could not be generically sepa- 

 rated ; and we now see that in the peculiar form and great size 

 of the scales the similarity is equally striking. 



