FOSSIL FISHES. 63 



In the "Bulletin of tlie Royal Academy of Belgmm" {^'Serie, t. XXVII. 

 p. 385), P. J. Yan Beneden describes the palate tooth of a iish, which is 

 ■without much doubt, generically identical with that on which the above 

 description is founded. It differs, however, from that here described in 

 having five tuberculated ridges, instead of four, on either side of the me- 

 dian line, in the number of tubercles on the ridges, and most of all in size ; 

 for the Belgian tooth is eight inches in diameter. M. Yan Beneden con- 

 siders his specimen as generically identical with a fish described by M. De- 

 Koninck and himself, in a preceding volume of the " Bulletin of the 

 Royal Academy of Belgium " ( 2° Serie, t. XYII., p. 143), and which was 

 made the type of a new genus {Palaedaphus). This remarkable fossil 

 was foimd in the Carboniferous limestone of Belgium, and was consid- 

 ered by the distinguished authors of the paper referi'ed to, as a portion of 

 the head and upper jaw of a Plagiostomous fish, having some resemblance 

 to Squatina. 



Excellent figures of both fossils were published by Yan Beneden and 

 De Koninck ; and judging from these and the minute descriptions which 

 they give, I am compelled to dissent from their view of the generic iden- 

 tity of their two species of Palaedaphus {P. Insignis and P. Devon- 

 iensis). 



The first seems to me, as to them, a portion of the head of a large Pla- 

 giostome, but the second exhibits characters which lead me to conclude 

 that it is the palate tooth of a Dipterian Ganoid, and that it belongs to a 

 genus that required a new name and description. ■ These I have ventured 

 to supply in describing the American specimen recently found ; uniting it 

 with the Belgian species (" Palaedaphus Dmoniensis "), in the genus 

 PEeliodus. 



My reasons for considering these the teeth of Ganoids and not of Se- 

 lachians are that they have essentially the structure of those of Dipterus, 



i. e., are composed throughout of true bone, and bear radiating ridges 



crowned with tubercles, of which the summits are coated with enamel — 

 and no such structure is known to exist in any Elasmobranch fish. In 

 all the members of this order the jaws are cartilaginous, and the teeth are 

 united to them by mere ligamentous attachments. 



If I am correct in separating generically the two species of PaZae- 

 daphus, that described by Yan Beneden under the name of Palae- 

 daphns Bevoniensis becomes Heliodus Devoniensis ; and if the view now 

 advanced in regard to the zoological relations of Heliodus is the true one, 

 we have in H. Bevoniensis by far the most gigantic member of the Dip- 

 terian family yet known, and one that must have rivalled in dimension 

 Dinichthys, the largest of the other great branch of the Ganoid order, the 

 Placoderms. 



