The Structure of Dinichthys 143 



SO. The upper margin of P?i comes in contact with PrO. There must apparently have 

 been some process (not found on the plaster cast) which fitted into a corresponding im^ 

 pression on the thickened fore part of PrO (See description of PrO on page 000). In front 

 we find an impression for the nostrils (ns). This part of P7<i touches the rostral, which 

 limits the other half of the nasal opening. The under margin of PJi is slightly curved with 

 some doubtful processes. Perhaps this part comes in contact with the antero'supra' 

 gnathal. On the inside there is nothing of special interest to see in the plaster cast. 



In the middle of P7\[ a sensory canal (sc) runs in a curve from the upper to the lower 

 margin. It forms the prolongation of the canal on PrO and connects it with the canal on 

 SO. This position of the sensory canal on P?S[ corresponds with what is known in other 

 Arthrodira. 



Post'Sub'Orbital. — This plate in other Arthrodira has been well known for a long 

 time. In Coccosteus it was described by Traquair in 1889 under the name of jugular or 

 post'maxillary. Woodward (1898) thought that it "occupies the position of an opercu- 

 lum." Jaekel in his reconstruction of Coccosteus in 1902 called this element "Quadrato- 

 jugale," and also figured it among Wildungen Arthrodira (1906-1907). Stensio in 

 1925 called this plate the Ps plate. 



In Dinichthys the PSO has never before been found and described. But both New- 

 berry (1889) and Claypole (1892.2) predicted its presence and we find it in Newberry's 

 reconstruction of Dinichthys. However, this prediction was soon forgotten, and in the 

 later papers on Dinichthys we never hear anything of this plate. The only traces of it are 

 to be found in Branson (1908-1911). His SO was very large. We have seen before, that 

 in the front it included the post-nasal. By a comparison of its posterior part with SO 

 from other specimens, it is not difficult to see that it is unusually large. It looks as if 

 another plate of triangular form were attached to the hind part of SO. But Branson could 

 not find the limit between them. 



Unfortunately, the material in the American Museum is also very incomplete. I 

 have been able to find only one fragment of this plate. It must have belonged to a large 

 specimen — perhaps to Dinichthys terrelli Nwb. (Plate IV, figures 8 and 9). As seen from 

 the plate, this fragment shows the natural margin only from two sides. This makes it 

 difficult to recognize the real form and size of PSO (Text-figure 26 A and B; Plate IV, 

 figures 8 and 9). But from Coccosteus, which in its construction is closely related to 

 Dinichthys, we know that PSO is of triangular form. The same is shown in Branson's 

 specimen. We must, therefore, suppose it to be the case in Diiiichthys also. The corner 

 between the natural margins in our plate must then be its upper posterior corner (Text- 

 figure 26 up). This supposition finds confirmation in a trace of an overlapping margin, 

 which is located in the opposite part on the outside of the plate (Text-figure 26 SO). 

 This must be the fragment of a limit between PSO and SO. (We know already that on 

 the inside of the hind upper corner of SO there is found a clear overlapping impression). 



When all these facts are taken into consideration, we can make a rather good picture 

 of this plate (Text-figure 26). It is a relatively thick, triangular plate with slightly convex 



