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Bashford Dean Memorial Volume 



Text-figure 19. 



Head roof of Coccosteus decipiens Ag. 



(Buffalo Museum specimen No. El 257 to show the 



presence of PM m this form.) 



tinue to MB (in Dinichthys terrelli Nwb. this 

 is usual). 



In addition to these two canals a third is 

 often to be found on C. It is the prolonged 

 canal from EB (Text-figure 12 VII). From 

 the limit between EB and C it continues up' 

 ward and nearly unites with the two previ' 

 ously mentioned canals in the center of the 

 plate. More infrequently there may be found 

 a very short piece of canal, going nearly 

 from the center of the plate in the direction 

 of the limit between C and M. Stensio 

 (1925) calls this canal "the middle head line" 

 (pit organ). It is very common in Coccosteus. 



Thus, in some specimens, from the cen- 

 ter of C five canals may radiate. The first 

 runs to PrO (Text'figure 12 I), the second to PtO (Text'figure 12 11), the third (pit organ) 

 in the direction of M, the fourth to EB (Text-figure 12 VIU), and the fifth to MB. 



As we have seen all the sensory canals on C radiate from the ossification center. It 

 is the case not only with the canals on C but also with all other sensory canals. Nearly 

 always they either begin from the ossification center or cross it, but never cross the ossi- 

 fication rays. If sensory canals separate into several branches, the dividing point is always 

 to be found in the ossification center of a plate. This phenomenon is very easily under- 

 stood if one considers the manner of growth of the plates. (See Heintz; 1929. 1, pp. 16-19). 



C overlaps all the plates which encircle it (MB, EB, M, PtO and PrO) except the 

 pineal. MB is much overlapped, but C only touches it along their limits. Therefore, as 

 mentioned before, we find a kind of cavity between the overlapped part of MB and the 

 underside of C (Text-figure 14). 



Post-Orbital. — This plate (Text-figures 12, 13, 14, and 15 PtO) together with M 

 forms the side margin of the head roof. This is a relatively long, narrow and very massive 

 plate. From the inside, it forms the median part of the lateral thickenings ot the roof, 

 and its inner margin coincides with the sharp inner limit of the latter. The neurocranial 

 process rises from the median part of this Hmit. The anterior part of PtO forms the 

 posterior half of the eye impression. PtO is overlapped by M and C and partly over- 

 lapped by PrO. 



On the outside of PtO, the sensory canal makes a three-branched figure. This figure, 

 like that on M, is very characteristic for PtO in all Arthrodira. The canal begins near the 

 post-orbital processes (Text-figure 12 III) and runs to the ossification center. There it 

 divides into two branches; one continues in the direction of C (III) and, as mentioned, 

 goes over onto these plates; the other branch runs down to M (IV). 



