RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 329 
A similar organ was described by Schmidt in Tremataspis, and 
considered by him to be a median nose. Such also is the view of 
Jaekel, who points out that a median 
pineal eye exists between the two 
lateral eyes in this animal, as in all 
other of these ancient fishes, so that 
this frontal organ does not, as Patten 
thinks, represent the pineal eye. The 
whole of this group of fishes, then, is 
characterized by the following striking 
characteristics :— 
1. Two well-marked lateral eyes 
nar the middle line Fic. 129.—NapiaL OPENING AND 
: LaTERaL Orpits oF Thyestes 
2. Between the lateral eyes, well- Verrucosus. (From Rouon.) 
marked median eyes, very small. 
3. In front of the eye-region a median orifice, single. 
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. 
STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT 
or AMMOC@TES. 
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 fibrille and star-shaped cells, 
arranged more or less in regular layers, with other sets of similar 
cells and fibrille arranged at right angles to the first set, or at vary- 
ing 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 
