LOWER OLD RED SANDSTONE OF SCOTLAND. 341 



Asterolepis was of great breadth, — greater proportionally than 

 even that of the ray ; and the middle of the palate was oc- 

 cupied by a thickly-tubercled, well-marked plate, of which I 

 liave possessed fragments for years, but whose place and form 

 I have only recently ascertained, by means of a magnificent 

 specimen now on the table, sent me by Mr Dick. [Spec. 15.] 

 That tendency in dermal tubercles to assume in some of the 

 ancient ganoids the form of teeth, and of teeth to assume in 

 some of the existing placoids the appearance of dermal sha- 

 gi-een, is a curious and surely not uninstractive circumstance ; 

 and it seems to throw light on some otherwise puzzling pecu- 

 liarities in the dental structure of ichthyolites, such as the 

 Coccosteus and Asterolepis. The teeth of Coccosteus, espe- 

 cially those placed so uniquely in the symphysis (of which 

 more anon), and all the ichthyic teeth of Asterolepis, seem 

 to be scarce less mere continuations of the osseous plates on 

 which they are based, than the external tubercles of these 

 same plates. A very 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 growth, wholly independent of 

 it ; 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 

 a? portions of jaws and palates, but they also represented 

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

 skin-covered jaws of the ordinary fishes. 



In referring to the jaws of the earlier ganoids, I would 

 first remark, that in the ordinary fishes the under jaw usual- 

 ly consists of four bones, — two on each side ; whereas in the 

 placoids, — sharks and rays, — it is composed, as in most of the 

 mammalia, of but two bones, — one on each side. In man 

 it forms but a single bone, which, however, in even the latter 



