The under part of the creature is still very imperfectly known: it had its central lozenge-shaped plate, like that on the under side of the _Pterichthys_, but of greater elegance, (see Plate III., fig. 3,) round which the other plates were ranged. "What an appropriate ornament, if set in gold!" said Dr. Buckland, on seeing a very beautiful specimen of this central lozenge in the interesting collection of Professor Traill of Edinburgh,--"What an appropriate ornament for a lady geologist!" There are two marked peculiarities in the jaws of the _Coccosteus_, as shown in most of the specimens, illustrative of the lower part of the creature, which I have yet seen. The teeth, instead of being fixed in sockets, like those of quadrupeds and reptiles, or merely placed on the bone, like those of fish of the common varieties, seem to have been cut out of the solid, like the teeth of a saw or the teeth in the mandibles of the beetle, or in the nippers of the lobster, (Plate III., fig. 4;) and there appears to have been something strangely anomalous in the position of the jaws--something too anomalous, perhaps, to be regarded as proven by the evidence of the specimens yet found, but which may be mentioned with the view of directing attention to it. "Do not be deterred," said Agassiz, in the course of one of the interviews in which he obligingly indulged the writer of these chapters, who had mentioned to him that one of his opinions, just confirmed by the naturalist, had seemed so extraordinary that he had been almost afraid to communicate it,--"Do not be deterred, if you have examined minutely, by any dread of being deemed extravagant. The possibilities of existence run so deeply into the extravagant, that there is scarcely any conception too extraordinary for nature to realize." In all the more complete specimens which I have yet seen, _the position of the jaws is vertical, not horizontal_; and yet the creature, as shown by the tail, belonged unquestionably to the vertebrata. Now, though the mouths of the crustaceous animals, such as the crab and lobster, open vertically, and a similar arrangement obtains among the insect tribes, it has been remarked by naturalists, as an invariable condition of that higher order of animals distinguished by vertebral columns, that their mouths open horizontally. What I would remark as very extraordinary in the _Coccosteus_--not, however, in the way of directly asserting the fact, but merely by way of soliciting inquiry regarding it--is, that it seems to unite to a vertebral column a vertical mouth, thus forming a connecting link between two orders of existences, by conjoining what is at once their most characteristic and most dissimilar traits.[M]