Some of the occipital bones of the _Holoptychius_ are very curious and very puzzling. There are pieces rounded at one of the ends, somewhat in the manner of the neck joints of our better known quadrupeds, and which have been mistaken for vertebræ; but which present evidently, at the apparent joint, the enamel peculiar to the outer surface of all the plates and scales of the creature, and which belonged, it is probable, to the snout. There are saddle-shaped bones, too, which have been regarded as the central occipital plates of a new species of _Coccosteus_, but whose style of confluent tubercle belongs evidently to the _Holoptychius_. The jaws are exceedingly curious. They are composed of as solid bone as we usually find in the jaws of mammalia; and the outer surface, which is covered in animals of commoner structure with portions of the facial integuments, we find polished and japanned, and fretted into tubercles. The jaws of the creature, like those of the _Osteolepis_ of the lower formation, were naked jaws; it is, indeed, more than probable that all its real bones were so, and that the internal skeleton was cartilaginous. A row of thickly-set, pointed teeth ran along the japanned edges of the mouth--what, in fish of the ordinary construction, would be the lips; and inside this row there was a second and widely-set row of at least twenty times the bulk of the other, and which stood up over and beyond it, like spires in a city over the rows of lower buildings in front. A nearly similar disposition of teeth seems also to have characterized the _Holoptychius_ of the Coal Measures, but the contrast in size was somewhat less marked. One of the most singularly-formed bones of the formation will be found, I doubt not, when perfect specimens of the upper part of the creature shall be procured, to have belonged to the _Holoptychius_. It is a huge ichthyodorulite, formed, box-like, of four nearly rectangular planes, terminating in a point, and ornamented on two of the sides by what, in a work of art, the reader would at once term a species of Chinese fretwork. Along the centre there runs a line of lozenges, slightly truncated where they unite, just as, in plants that exhibit the cellular texture, the lozenge-shaped cells may be said to be truncated. At the sides of the central line, there run lines of half lozenges, which occupy the space to the edges. Each lozenge is marked by lines parallel to the lines which describe it, somewhat in the manner of the plates of the tortoise. The centre of each is thickly tubercled; and what seems to have been the anterior plane of the ichthyodorulite is thickly tubercled also, both in the style of the occipital plates and jaws of the _Holoptychius_. This curious bone, which seems to have been either hollow inside, or, what is more probable, filled with cartilage, measures, in some of the larger specimens, an inch and a half across at the base on its broader planes, and rather more than