Palaedaphus, Holodus, and Clieirodus. 15 



quite sure that in Palcedaphus insignis we have the snout or 

 upper part of a fish-head ? that its dental plates belong to the 

 upper jaw ? that the lateral excavations are really nasal 

 openings ? 



The answer to these questions will be found by comparing 

 PaloidapJius insignis not with the snout, but with the loiver 

 jaw of Dipterus. 



The smooth enamelled surface seen in fig. 6 corresponds 

 obviously with the lower aspect of the united dentaries of 

 Dipterus shown in fig. 3 ; and in the cast there is a distinct 

 trace of a suture in a situation analogous to that which in 

 Dipterus separates the dentary from the angular behind. The 

 labial margins are very similar, though that of Dipterus is 

 more convex in its contour when seen from above or below 

 (figs. 2, 3). The lateral excavations in Palondaplius in- 

 signis (?/, figs. 5 & 7) correspond exactly with those below 

 the hinder part of the enamelled lower lip of Dipterus (figs. 3, 

 4) , as will be at once apparent on comparing the lateral view 

 of the lower jaw oi Dipterus (fig. 4) with that oi Palcedap)hus 

 (fig. 7) ; these excavations are therefore not nasal openings. 

 The ridged plates supposed to be the halves of the upper jaw 

 of Paloidaplius insignis represent the splenial dental plates of 

 Dipterus^ from which they differ in their relatively more an- 

 terior position and in coming into contact with each other in 

 front. 



There is therefore, in my mind, not the smallest doubt 

 that Paloidaphus insignis of Van Beneden and De Koninck is 

 the symphysial part of the lower jaw of a gigantic Ctenodo- 

 dipterine fish, and not the snout of a Plagiostome as has been 

 supposed. The same interpretation, size only excepted, must 

 necessarily be accepted for Holodus Kiprijanoioi of Pander. 



Paloidaphus devoniensis is evidently the upper or palatal 

 plate of an allied species ; there is therefore no necessity for 

 altering its name to Heliodus ; and if Prof. Newberry's //. 

 Lesleyi does belong to the same genus, the name Heliodus must 

 drop. 



III. Cheirodus, M'Coy. 



A small dental plate from the Carboniferous Limestone of 

 Derbyshire was described and figured by M'Coy under the 

 name of Chirodus pes-ranoi *, and considered by him to be a 

 tooth of a Selachian allied to Ceratodus, which was at that 

 time still reckoned amongst the sharks. 



* Ann. & Mag. Nat. Hist. (2) ii. 1848, pp. 130, 131 ; * Palteozoic 

 Fossils,' p. 616, pi. Bg. fig. 9. The name is spelt "Chirodus" by M'Coy ; 

 but, in common with the majority of authors, I prefer the form "Cheiro- 

 dus." 



