152 Bashford Dean Memorial Volume 



developed joint with a joint head on the one side and joint socket on the other. The im- 

 pression of PIG is very flattened and quite irregular in shape. It is not placed on the 

 upper hind margin of the jaw, as is usual, but on the side margin. The thickenings of 

 PSO also bear an impression instead of a condyle. It is not placed on the lower margin 

 of PSG but in the middle of its interior. In all probability PIG was attached to PSG 

 only by ligaments, muscles and connective tissue (Adams, 1919). A real joint with condy- 

 loid articulation has not been developed here. The lower jaw was suspended to the upper, 

 but not articulated with it. 



This discovery makes it very difficult to understand in what manner the heavy and 

 massive functional portion of the lower jaw could effectively work against the upper jaw. 

 The thin and weak blade in itself is an unsatisfactory carrier for such a strong tooth com- 

 bination.^ Now we know that the joint also was very primitively and unsubstantially 

 built. The functional portion of the lower jaw could not have been satisfactorily used if 

 its movements were based on the same principle as in other vertebrates. We must wait to 

 discuss this question further in a later section when we shall study the structure of the 

 body carapace. 



I have now described all the hitherto known side plates of the head. But in the 

 hterature descriptions of some other plates belonging to the same region can be found. 

 Thus Claypole in 1892.1 mentioned that he had found in Dimchthys one pair of inter- 

 mandibular plates and one ''scalpriform tooth" between the two pre-maxillar plates, 

 apparently working against the two inter-mandibular plates. He thinks that probably 

 these two plates were also developed in other Arthrodira. 



Hussakof (1909) described a symmetrical dental plate from a new form, Dinognathus 

 ferox Huss. He thought that it either represented the fused ''pre-maxillaries" or was a 

 "pre-mandibular" placed at the symphysis. Perhaps in reality it was a median inter' 

 maxillary plate, corresponding to that described by Claypole. 



Jaekel in 1919 described three other plates placed between the IG, but on the 

 underside. He thought that one pair was the ossified fore part of otherwise cartilaginous 

 "Hyoidbogen.'' Together with a small median plate — copula — they were placed between 

 the front of both "mandibles" (IG). Probably these three plates correspond to the plate 

 in Dinichthys mentioned by Claypole. No trace of homologous elements is known from 

 other Arthrodira. 



Lastly, we shall now see how the single plates of the side part of the head are con- 

 nected. In Text-figure 34 a reconstruction of this part is given from the outside (A) and 

 from the inside (B). The drawing in itself is so clear that detailed description is not neces- 

 sary. The outHne and relative si2;e of PSO are taken mostly from Branson's figures of the 

 sub-orbital (1908.1). The size and form of P7^ are more or less doubtful. The front part 



"Some authors (Newberry 1873; Claypole 1893.5; Hussakof 1906; Eastman 1907.2; Woodward 1922) express the opinion that 

 the blade of IG was, perhaps, strengthened by cartilage. This in my opinion is improbable; the blade is convex on both sides and no 

 trace of contact with cartilage is shown. 



