RELATIONSHIP OF AMMOCCETES 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 W 



characterized by the following striking , : -'-i m ? ;S ^r ' 



characteristics : — 



1. Two well-marked lateral eyes 



,, .,,•, -,. Fig. 129. — Naeial Opening and 



near the middle line. T m ^ „ „ m7 . 



Lateral Orbits of Thyestes 



2. Between the lateral eyes, well- Verrucosus. (From Rohon.) 

 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 



of Ammoccetes. 



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 Eohon's paper already referred to. It 

 is, so he describes, clearly composed of fibrillae and star-shaped cells, 

 arranged more or less in regular layers, with other sets of similar 

 cells and fibrilke 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 



