The Structure of Dinichthys 



157 



factory curvirtg of this part, so as to fit the head roof together with the side plates, we 

 must admit that the plates in the front region are not only crushed, cleft and slid, but 

 flattened and deformed by pressure. We may call this kind of deformation microstructural. 

 The plate in such a case is apparently not destroyed; it shows no clefts or breaks. But 

 under enormous pressure it was flattened and became thinner and larger. This deforma' 

 tion can be perceived only by study of sections under the microscope. The whole bone 

 becomes more or less laminated in character. The Haversian canals, placed parallel with 

 the surface of the plate, are flattened; those placed obliquely or perpendicularly to 

 it are enlarged. All the bone-cell cavities are also flattened. Good pictures of such 

 a microstructurally deformed bone are to be found in Heintz, 1929.1 (PI. XXII, fig. 3; 

 PI. XXIV, fig. 4). 



In reconstructing the curve of the front part of the head roof, the position of SO is 

 of aid. As we have seen, on the front part of the handle of SO there were some ridges, 

 which served to attach SO to the pre-orbital process of the head roof. On the latter were 

 also developed some sockets and ridges. Text'figure 39 shows the fore part of SO and 

 the pre-orbital process. It demonstrates how well the single processes and sockets were 

 adapted to one another and how intimately these two parts were connected. But if the 

 handle of SO is attached to the PrO of a compressed specimen, the hind part of it, the blade, 

 stands far out from the side margin of 

 the head roof (Text-figure 40). Mean- 

 while, we know that the upper fore cor- 

 ner of the blade should touch a notch 

 behind the post-orbital process and the 

 edge of the blade should run along the 

 side margin of the head roof. 



To attain such a position of SO, it 

 is not enough to curve the handle, the 

 front part of the head also must be strongly 

 bent and shrunk. The front margin be- 

 tween the pre-orbital process and R, 

 instead of being placed nearly perpendic- 

 ularly to the median axis of the head, 

 must be strongly curved. The point of 

 the pre-orbital process is now not di- 

 rected sideways but backwards. This 

 gives the front margin of the head a nearly 

 half-circular outline, instead of a nearly 

 straight line. 



The rostral in the reconstructed 

 head roof has also greatly changed its 

 position. Instead of extending forward. 



Text-figure 39. 



The contact between the head roof and the 



sub-orbital plate. 



A, inside view — SO is separated from PrO to show 



clearly sockets and ridges; B, outside view — SO is in 



close contact with the head roof. 

 Orb, orbit; P, pineal; PrO, pre-orbital; R, rostral; SO, sub-orbital 



