[Footnote M: These statements regarding the character of the teeth and the position of the jaws of the _Coccosteus_ have been challenged by very high authorities. I retain them, however, in this edition in their original form, as first made nearly six years ago. In at least two of my specimens of Coccosteus the teeth and jaw form unequivocally but one bone--a result, it is not improbable, of some after anchylosing process, but which still solicits inquiry as not yet definitely accounted for. The matter of fact in the case is certainly one which should be determined, not analogically, but on its own proper evidence, as furnished by good specimens. As for the remark regarding the probable position of the creature's jaws, it was ventured on at first, as the reader may perceive, with much hesitation, and must now be regarded as more doubtful than ever. Its repetition here, however, will, I trust, be regarded as simply indicative of a wish on the part of the writer, that the question be kept open just a little longer, and that further examination be made. There is certainly something very peculiar about the mouth of the Coccosteus not yet understood, and singularly formed plates, connected with it, which have not been introduced into any restoration, and the use of which in the economy of the animal seem wholly unknown. [1850.--I have at length found a very perfect specimen of the nether jaw of _Coccosteus_, and am prepared to show that it was of a character altogether unique. It had its two groups of from six to eight teeth, (exactly where, in the human subject, the molars are placed,) that seem to have acted on corresponding groups in the intermaxllaries, and two other groups of from three to five teeth placed at right angles with these, direct in the symphysis, and that seem to have acted on each other. But though these unique teeth of the symphysis formed a vertical line of mouth, it joined on at right angles to a transverse line of the ordinary type, as the upright stroke of the letter T joins on to the horizontal line a-top.] Fourth Edition.]