Bashford Dean Memorial Volume 



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Text-figure 37- 



(Above) 

 Schematic drawings to show how the head of Dinichthys 

 may be crushed under pressure. 



Text'figure 38. 



(R.ght) 



Sections of the head roof in the region of the neurocranial process: A, crushed; B, in reconstruction. 



C, central; NC, neurocranial cavity; >{fi, neurocranial process; P, pineal; PtO, post-orbital. 



inwards. In the schematic drawing, Text-figure 37, we see that if the head roof was placed 

 with the inside up (A) or down (B) the result of the pressure was the same. The side 

 margins have a more horizontal position, the central part is curved inward. As a rule it 

 broke along the median line of the head, the limit between both centrals. 



This pressure phenomenon explains why we always find the head root of Dinichthys 

 with the central part bent inwards. In all the reconstructions and models of Dinichthys 

 known to me this bending has not been corrected. Therefore the head has been given a 

 flat or concave roof. The pressure also influenced the position of the neurocranial 

 processes. In crushed specimens they are more vertically placed and the distance between 

 their points is much larger than in reality. Besides, they are often. pressed into the head 

 roof which becomes swollen on the outside where the processes are attached. If we cor' 

 rect all these disturbances and reconstruct the original curve of the head and position of 

 the processes, we shall see that the place in front of the neurocfanium is quite large and 

 deep. Such a reconstruction is made in the half-schematic Text-figure 38. It shows the 

 section of the head in the region where the neurocranial processes were situated. 



The most difficult problem is to reconstruct the original curve of the front part ot 

 the head roof. If we look at the half-schematic drawings of some Dinichthys head roofs 

 given in Text-figure 36, we can easily see that all specimens are either clett from the central 

 part forward to the rostral region, or that more or less large portions of the front part ot 

 the head are absent. The arrangement of the clefts indicates clearly that the front part 

 of the head was not only bent from side to side, but also from back to front. Looked at 

 from the inside it must have formed the half of a bowl-shaped impression. Besides the 

 clefts, the front part is also loosened along the sutures of the plates. Under pressure the 

 single plates slid from each other along the sutures, without losing contact with one 

 another. Such loosened sutures are often to be found between PrO, P and R. 



The clefts and slides along the limits are, however, not enough to explain the flat- 

 ness of the front part of the head in fossil specimens. To get an approximately satis- 



