134 Bashford Dean Memorial Volume 



characteristic for the M on all Arthrodira. The center of this figure is placed in the ossifi- 

 cation center of the plate. One branch goes up to the PtO (IV), the other (V) down and 

 outward to the external angle (Ex) of the roof, and the third (VI) runs over on EB where 

 it continues into the previously mentioned canal. 



Post-Marginal. — This plate (Text-figures 12, 13, 14, 15 PM, 1? and 18; Plate 

 III, figure 7) is one which has never before been observed in Dinichthys. It is a little 

 triangular plate that forms the extreme point of the external angle and gives to it its char- 

 acteristic sharp-pointed outline. Like a little hollow cap, PM is fitted on to the extreme 

 parts of M and EB and in this manner connects them firmly together. On the inside PM 

 greatly overlaps M and EB; on the outside more moderately. The suture on the outside 

 is complicated, as is seen from Text-figures 12, 17, 18 and Plate III, figure 7- PM is the 

 plate of the whole head roof most loosely joined to its neighboring plates, and therefore is 

 often found isolated. For the same reason the otherwise complete head roof often lacks 

 PM only. 



In other Arthrodira PM has been found and figured before. Thus Traquair in 

 1894, in describing a head of Phlyctaenaspis acadica Wt. from Canada, figured a new plate 

 which formed the extreme corner of the roof and was placed behind M and EB. He called 

 it "angular." Considering its position, relation to other plates, and the fact that we find 

 on it a continuation of the sensory canal from M, this plate corresponds with our post- 

 marginal. 



Jaekel in his papers on Pholidosteus (1907) gives some reconstructions of this form. 

 Here we find a small plate between M, EB, and PSO. This plate, considering its position 

 and relation to other plates, must have been the same as PM in Dinichthys. Jaekel has 

 never described it nor given it a special name. In the reconstruction of some other 

 Arthrodira from Wildungen (1906.2) we also find some plates in the posterior part of the 

 head which must correspond with PM. 



The present writer has described (1930.2) in Heterostius sp. a little plate placed on the 

 extreme angle of the head roof. It was called angular, corresponding to the name given by 

 Traquair to the same plate in Phlyctaenaspis. There is, however, no doubt that we have 

 here PM. It is placed beside M and EB, and the sensory canal from M continues on it. 



In Coccosteus, PM has never been 

 described. In Traquair's (1890.2) and 

 Woodward's (1891.1) reconstructions it 

 is absent. Jaekel's (1902) reconstruction 

 shows a plate between the sub-orbital, 

 post-sub-orbital and marginal. He called 

 it "Supra-temporale." Perhaps it is a 

 misplaced PM, but from its position and 

 Text-figure 17- relation to other plates it is, rather, an 



The post-marginal plate of Dinichthys intermedius 



Nwb.: a, from outside; b, from inside. entirely new element. The present _ 



M, position of marginal; EB, position of externo-basal. writer, when Studying in the Buffalo 



