The Structure of Dinichthys 



151 



described in detail in Diyiichthys. The first author to indicate its existence in the Arthro' 

 dira was Eastman (1906.1) who described and figured it in Dinomylostoma, correctly in' 

 dicating that it "forms a hinge [for attachment] with the suspensorial cartilage of the 

 cranium." In 1907, Jaekel described and figured it in Pholidosteus. He called it "Articu' 

 lare" to correspond with the similar element in higher fishes (Text-figure 33). In many 

 later papers he mentions this plate and has also published some drawings of it (1919, 1925, 

 1929). In Dmichthys the plate was, for the first time, mentioned and schematically figured 

 by Adams (1919) who gave only a very short description of it. 



PIG is a small, triangular plate, very solidly attached to the hind rounded part of the 

 IG blade. On the outside it overlaps the blade strongly and extends forward in a long, 

 sharp tongue (Text-figure 30 C). On the inside it covers IG a little and forms a narrow 

 sheath over the hind part of the blade (Text-figure 31 C). The surface of PIG is corru- 

 gated and rough. Its hinder part is thickened and on it is an oblong, irregular impression 

 (Text-figures 30 and 32 im). PIG gets thinner forward and is very thin along the Hmit of 

 IG. It is remarkable that this plate has been found only twice, attached to a pair of IG 

 belonging to the same individual. It looks as if PIG were very strongly connected with 

 IG. But, in spite of this, not even an impression or overlapping trace is to be found on the 

 hind part of the lower jaw. 



In the collection of the American Mu- 

 seum, among 10 to 20 specimens only two or 

 three lower jaws were found showing a very 

 indistinct trace of an overlapping on the hind 

 part. In the Buffalo Museum only one ex- 

 ample (No. E2033, described by Hussakof 

 and Bryant, 1918, pp. 34-35, Fig. 6) showed a 

 doubtful plate in the hind part of a jaw blade, 

 perhaps a fragment of PIG. In other Arthro- 

 dira this plate, as we have seen, is only known 

 in some forms from Wildungen (Jaekel), and 

 in Dinomylostoma (Eastman). In my opinion 

 all this indicates the possibility that PIG in 

 the majority of Arthrodira has either not been 

 ossified but was formed from cartilage, or 

 that only its surface was ossified. This sup- 

 position also explains the rough and wrinkly 

 surface of PIG. 



With the help of PIG, IG was con- 

 nected to PSO. This movable articulation , Text-figure 33. 



„ 1 -..1 ^1 • ■ ■ ^ ■ 11 I Jaekel's reconstruction of the jaw apparatus in 



corresponds with the jaw joint in all other dl ij at ■ -j d r ■ j 



, T 1 1 ■ 1 fholidosteus: A, trom outside; B, rrom inside, 



vertebrates, as Jaekel pointed out in 1907- 



■wj J r- 1 1 • 11 '^> Jaekel's "Spleniale"; PIG, Jaekel's "Articulare"; PSO, 



But we do not find here, as is usual, the well post-sub-orbitai; so, sub-orbital. 



