84 THE ASTEROLEPIS, 



scarce less mere continuations of the osseous plates on which 

 they are based, than the external tubercles of these same 

 plates. An entirely dissimilar state of things obtains among 

 our ordinary fishes of the present time : the teeth are of a 

 different formation from the bone on which they rest, and, 

 in at least their earlier stages of gi'owth, wholly independent 

 of it j but be it remembered that, as in the existing placoids 

 teeth and shagreen are alike of dermal origin, so in not a 

 few of the ancient ganoids teeth and tubercles were alike of 

 dermo-osseous origin. The plates on which they grew acted 

 as portions of jaws and palates ; but they also represented 

 skin, and differed very materially, in consequence, from the 

 skin-covered jaws of the earlier ganoids. 



Of the upper maxillary bones of the Asierolepis, I only 

 know that a considerable fragment of one of the pieces, re- 

 cognised as such by Agassiz, has been found in the neigh- 

 bourhood of Thurso by Mr Dick, unaccompanied, however, 

 by any evidence respecting its place or function. It exhibits 

 none of the characteristic tubercles of the dermal plates, but 

 is simply a long bent bone, resembling somewhat less than the 

 half of an ancient bow of steel or horn, — such a bow as that 

 which Ulysses bended in the presence of the suitors. By some 

 of the Russian geologists this bone was at first regarded as a 

 Fig. 38. 





MAXILLAKT BONE ? 



(One-fourth nat. size, linear.; 



portion of the arm or wing of some gigantic Pterichthys. In 

 the accompanying print (fig. 38), I have borrowed the general 



