[Footnote 46: The above descriptions of the scales and teeth of these two fishes, are taken from specimens in my own collection. None of my holoptychian scales show incontestable the proportion of the covered to the exposed part. Judging from the aspect of one of them, the wrinkled portion occupied perhaps about three-fifths of the entire scale, the remaining part being covered by the overlapping edges of those adjacent; for the characteristic corrugated surface was essentially an external ornament, and ceased at the point where the external bone passed into the interior. I may remark, that the upper side of the scales is not very frequently seen in the Burdiehouse limestone, the rough surface usually adhering to the rock, and leaving only the smooth inner side exposed. Out of seven specimens from that locality, only one shows the upper side, and that by no means in a perfect state of keeping. The structure alike of scales and bones can be seen to much greater advantage in the shales, ironstones, and coals of the coal-fields, where, owing to the soft nature of their matrix, the fossils can be readily cleared and exposed.] But perhaps the most remarkable and characteristic parts of the carboniferous holoptychius were its jaws and teeth. As we might readily conjecture from the great size and strength of the scales and cranial plates of this fish, its dentition was of a correspondingly massive type. The under jaw, with the usual corrugated ornament, frequently exceeded a foot in length, and displayed along its upper edge a thick-set group of teeth. Of these there were two kinds one of a smaller size and more blunted form, with short indented furrows at their base; the other of a greatly more formidable size, grouped at intervals among the smaller ones. The front end of each under jaw bore one of these long conical tusks, serving as it were to guard the entrance of the mouth. Each of the larger teeth had a base strongly marked with longitudinal furrows, and sank deep into the jaw, with the bone of which it sometimes anchylosed.[47] The part of the tooth above this socket had an oval form, so flattened as to present two cutting edges, one facing the front, the other the back of the mouth, and meeting at the upper end of the tooth which was sharp and pointed. Such large conical tusks may frequently be obtained, having a length of two or three inches, while occasionally they range as high as six or seven, the smaller teeth seldom reaching so much as an inch. It is difficult to see how, with such a formidable dentition, the jaws could readily close. In some specimens I have seen deep hollows beside the bases of the teeth, which may possibly have received those of the opposite jaw, but the gigantic tusks at the entrance of the mouth seem to have stood high over the jaw, passing outside like those of the wild-boar. If this be correct, the jaw of the holoptychius would unite the mechanism of both the alligator and the crocodile--its recipient