[17] In all probability it is likewise the principle of the placoid skull. The numerous osseous points by which the latter is encrusted, each capable of increase at the edges, seem the minute bricks of an ample dome. It is possible, however, that new points may be formed in the interstices between the first formed ones, as what anatomists term the _triquetra_ or _Wormiana_ form between the serrated edges of the lambdoidal suture in the human skull; and that the osseous surface of the cerebral dome may thus extend, as the dome itself increases in size, not through the growth of the previously existing pieces,—the minute bricks of my illustration,—but through the addition of new ones. Equally, in either case, however, that essential difference between the placoid skull and the placoid vertebra, to which I have referred, appears to hinge on the circumstance, that while the osseous nucleus of each vertebral centrum could form, in even its most complicated shape, from a _single_ point, the osseous walls of the cranium had to be formed from _hundreds_. The accompanying diagram serves to show after what manner the vertebral centrum in the Ray enlarges with the growth of the animal, by addition of bony matter external to the point in the middle, at which ossification first begins. The horizontal lines indicate the lines of increment in the two internal cones which each centrum comprises, and the vertical ones the lines of increment in the lateral pillars. [Illustration: Fig. 23. SECTION OF VERTEBRAL CENTRUM OF THORNBACK.] [18] One of the Thurso coprolites in my possession is about one fourth longer than the larger of the two specimens figured here, and nearly thrice as broad. [19] In two of these, in a collection of several score, I have failed to detect the spiral markings, though their state of keeping is decidedly good. There are other appearances which lead me to suspect that the _Asterolepis_ was not the only large fish of the Lower Old Red Sandstone; but my facts on the subject are too inconclusive to justify aught more than sedulous inquiry. [20] The shaded plate, (_a_,) accidentally presented in this specimen, belongs to the upper part of the head. It is the posterior frontal plate F, which half-encircled the eye orbit, (see fig. 29;) and I have introduced it into the print here, as in none of the other prints, or of any other specimens, is its upper surface shown. [21] The late Mr. John Thurston.