In addition, behind the eye-region a median plate is always found, frequently different in structure to the rest of the head-shield, being harder in texture--the so-called post-orbital plate. [Illustration: FIG. 129.--NARIAL OPENING AND LATERAL ORBITS OF _Thyestes Verrucosus_. (From ROHON.)] STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES. What is the structure of this head-shield? It has been spoken of as formed of bone because it possesses cells, being thus unlike the layers of chitin, which are formed by underlying cells but are not themselves cellular. At the same time, it is recognized on all sides that it has no resemblance to bone-structure as seen in fossil remains of higher vertebrates. The latest and best figure of the structure of this so-called bone is given in Rohon's paper already referred to. It is, so he describes, clearly composed of fibrillæ and star-shaped cells, arranged more or less in regular layers, with other sets of similar cells and fibrillæ arranged at right angles to the first set, or at varying angles. The groundwork of this tissue, in which these cells and fibrils are embedded, contained calcium salts, and so the whole tissue was preserved. In places, spaces are found in it, in the deepest layer large medullary spaces; more superficially, ramifying spaces which he considers to be vascular, and calls Haversian canals; the {330}star-like cells, however, are not arranged concentrically around these spaces, as in true Haversian canals. This structure is therefore a calcareous infiltration of a tissue with cells in it. Where is there anything like it? As soon as I saw Rohon's picture (Fig. 130), I was astounded at its startling resemblance to the structure of muco-cartilage as is seen in Fig. 131, taken from Ammocoetes. If such muco-cartilage were infiltrated with lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be preserved in the fossil condition, and be comparable with that of Cephalaspis, etc. [Illustration: FIG. 130.--SECTION OF A HEAD-PLATE OF A CEPHALASPID. (From ROHON.)] [Illustration: FIG. 131.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE OF AMMOCOETES.]