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Bashford Dean Memorial Volume 



forms a deep, well-limited cavity, which was occupied by the fore part of 

 the cartilaginous neurocranium. 



The postero'lateral part (PL) makes a long deep impres- 

 sion, which opens backward between the external angle of 

 the roof and the joint process. It is limited in front by the 

 connecting bridges (CR). This limit is not especially 

 sharp. Nearly from its center runs a fine thin ridge, 

 which divides the latter part of the postero-lateral im- 

 pression in two (Ir). The limit of the lateral thicken- 

 ings is very high and forms a steep wall (Rd). The 

 Hmit of the posterior consolidating arch is not so 

 high, but is well marked. Probably this long deep 

 fossa was occupied by strong muscles. Their di- 

 rection and function will be discussed later. 



After having described the general out 

 line of the whole head roof, I will now try to 

 give a description of the single plates which 

 compose it. But it is not always easy to 

 find the limit between the single plates, 

 and it is therefore difficult to define clear- 

 ly the overlappings between them. The 

 best and most careful description of the 

 single plates of Dinichthys intermedins 

 Nwb. and of their relation to one another 

 was given by Claypole in 1892. How- 

 ever, the results of my investigation do 

 not always correspond with those of 

 Claypole. 



I have found in other Arthrodira 

 (especially clearly developed in Hoynosti' 

 us) that there is a law governing the manner in which the single plates overlap one another. 

 From MB as center (Text-figure 9) all the posterior plates overlap the plates in front of 

 them or at their sides. For example, MB overlaps EB and C, C overlaps M and PtO, etc. 

 In other words, the-plates are placed nearly in the same way as tiles on a roof. This principle 

 of arrangement we also find in the building of the body carapace, and not only in Arthro- 

 dira but also (with some exceptions) in the Asterolepida. In larger forms with a thick 

 armor, this regularity may be more or less disturbed by the very complicated structure of 

 the sutures. Such is the case with Dinichthys. 



Before describing the single plates it must be noted that in the Arthrodira the in' 

 dividual variation in outline, si^e and position of the single plates is perhaps greater than 



Text-figure 15. 

 Limits between the single plates of the head roof of 

 Dinichthys intermedins Nwb. : outside one in continu- 

 ous lines, inside ones dotted. 



C, central; EB, externo-basal, M, marginal, MB, median-basal; P, 



pineal; PM, post-marginal; PrO, pre-orhital; PtO, post-orbital; 



R, rostral. 



