92 REPORT—1850. 
sharks we find teeth taking exactly the form of the external shagreen. This is the 
case in the angel fish (Squatina), the whole of whose palate within the teeth proper is 
covered by a shagreen undistinguishable from that which covers its back. The same 
peculiarity occurs, too, in the mouth of the Port Jackson shark (Cestracion), imme- 
diately within the pavement teeth. Now, in the palate of the Dipterus, we find, ap- 
parently on a similar principle, the dermal enamel of the external plates and scales of 
the creature completely reproduced. The skin within the mouth, if one may so speak, 
completely corresponds with the skin outside. We find it bearing the same rich gloss, 
and punctulated by the same thickly-set microscopic tubes. There occurs what 
seems to be a similar reproduction of dermal peculiarities within the mouth of the 
Asterolepis. Immediately behind its reptile teeth we find the surface roughened on 
a base of bone with osseous tubercles, that in some places seem elongated into squat 
teeth, and in some assume the characteristic stellar shape, but which present ge- 
nerally the appearance exhibited by the tubercles which fret the external plates of 
the cranium, especially as these appeared in the young animal, and ere they attained 
to their normal star-like form, This tendency of tubercles to assume the form of 
teeth in the ancient Ganoids, and of teeth to assume in some of the existing Placoids 
the appearance of shagreen, seemed a curious and not uninstructive circumstance ; 
and threw light on some otherwise puzzling peculiarities in the dental structure of 
ichthyolites, such as the Coccosteus and Asterolepis. The teeth of the former, 
especially those placed so uniquely in the symphysis, and all the ichthyic teeth of the 
latter, seem to be nearly as much mere continuations of the osseous plates on which 
they are based, as the external tubercles of these same plates. A very different 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; 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 jaws or portions of jaws, but they also re- 
presented skin, and differed very materially, in consequence, from the skin-covered 
jaws of the ordinary fishes. Mr. Miller had lately succeeded, he said, in procuring 
a cerebral plate of the Asterolepis, that presented one of the sockets in which the 
under jaw of that animal had acted, and which in one respect resembled the sockets 
of the jaw of the Lepidosteus of America,—one of the few reptile fish which still 
continue to exist. It contained two antagonistic processes, placed on opposite sides 
of the socket ; on one of which the condyles had acted every time the jaw opened 
or shut ; whereas the other merely served as a check, which prevented the jaw from 
opening beyond a certain width. In the under jaw of a large Thurso ichthyolite of 
the reptile family, still unnamed and undescribed, and some of whose internal bones 
could not at the present stage be distinguished from those of Asterolepis, the under 
jaw seemed to consist of five pieces,—first of a central key, like that of the Dipterus 
in its foetal state; next, of two side bones, that were united to it; and lastly, of two 
slender bones, that seemed to occupy a similar place in the under jaw of this fish that 
the intermaxillary bones do in the upper jaws of most other fishes. 
Mr. Miller next proceeded to describe the under jaw of the Coccosteus as one of the 
most extraordinary of the period, or perhaps of any period. It consisted of two bones, 
one on each side, which were furnished each with its group of from six to eight teeth, 
placed exactly where in the human subject the molars occur. And these groups seem 
to have acted against corresponding groups in the upper jaw. But at right angles with 
these molar groups, exactly in the medial symphysis, there was another group of teeth 
from three to five in number ; and these could not have acted against teeth placed in 
the upper jaw, but were directly opposed,—the terminal group in the one bone of the 
nether jaw to the terminal group in the other bone. Mr. Miller stated, that about nine 
years ago he had called the attention of palzontologists to something very peculiar in 
the jaws of the Coccosteus, and had solicited inquiry respecting them ; but the restora- 
tion of Agassiz, who had been misled by imperfect specimens, several of them derived 
from Mr. Miller’s own collection, had been regarded as settling the point the other 
way; and it was only during the last season that Mr. Miller was enabled to demon- 
strate, from newly-found specimens, that the peculiarity had really existed. One of 
these specimens he owed to a lady of Cromarty (Mrs. James Hill), an intelligent geolo- 
gist and successful collector. The teeth of the Coccosteus, viewed as prepared trans- 
