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margins. It limits the hind margin and upper posterior corner of SO, and is probably to be 

 placed so as to prolong SO mostly backward. Thus it fills up the space between the hind 

 corner of SO and the external angle of the head roof. Its size must be quite variable in 

 different species, all in proportion to the size of SO. In Dinichthys terrelli Nwb., where SO 

 is relatively narrow and goes farther back, PSO is smaller in contrast to Dinichthys inter- 

 medius Nwb. with short and broad SO where it must have been much larger. 



Text-figure 26. ' • 

 The post-sub-orbital plate of Dinichthys ? terrelli Nwb. : A, from outside; B, from inside. 



so, overlapping margin covered by SO: tc, thickened part in center of inside serving as attachment for PIG; up, upper hind corner 



of PSO. 



The most interesting point in the structure of PSO is a thickening placed nearly 

 in the middle of its inner side (Text-figure 26 B tc; Plate IV, figure 8). It is rather large 

 with a rough surface of horseshoe shape. If we place PSO as proposed before, the curving 

 point of the horseshoe will be directed upward to the upper corner of the plate and the 

 open part to SO. To this remarkable structure the lower jaw was movably attached. 

 It has never been observed in Coccosteus — probably because PSO in this form is so 

 small that it is difficult to study it in detail. But Jaekel in 1907 and 1925 (1925.1) in a 

 reconstruction of Pholidosteus has described it as a "cartilage" placed on the underside of 

 PSO (JaekePs "Quadra to-jugale"). He supposed it was the "Quadratum." 



The two other elements of the upper jaw are most commonly known under the 

 names of the maxilla and pre-maxilla. Besides being used to designate mouth bones of 

 the fishes, they were given to different plates in the Arthrodira themselves. Traquair 

 called the post-nasal the pre-maxilla and the sub-orbital the maxilla. Newberry gave the 

 same names to the "teeth" of Dinichthys. It is better to adopt the names proposed by 

 Hussakof, though they are a little cumbrous. 



