The Structure of Dmichthys 



175 



Intero-Lateral. — This plate (Text-figure 62; Plate VIII, figures 20 and 21) forms 

 the front part of the hind wing of Newberry's "clavicular," as Eastman, Hussakof and 

 Smith have correctly pointed out. The only description of this (and it a short one) was 

 given by Smith (1909 and 1910) in papers on the ventral shield of Dmichthys hohnodeus 

 CI. According to Smith it resembles the IL in Coccosteus. 



In other Arthrodira it has long been known. Pander (1857) figured it in Coccosteus.; 

 Traquair described it in the same form (1890.4), and later in Phlyctaenaspis (1893); 

 and also Jaekel in 1903.2 described IL correctly in Coccosteus, calling it the "Jugulare." 

 In 1907, in describing PhoUdosteus, he mentioned IL in this form as belonging to the lower 

 jaw, and called it "Angulare." He believed that: "Die Angulare dem Bauchpanzer ent- 

 nommen, erst sekundar in den Verband des Unterkiefers gezogen ist." In reality, as Dean 

 pointed out in 1908, Jaekel here made a mistake. His "Angulare" is IL, which in some 

 crushed specimens was displaced and moved near the blade of the lower jaw (Text-figure 

 63). But in many other specimens, as the present writer was able to discover in examina- 

 tions of Jaekel's collection in 1927, this plate was in contact with AVL. It fits so perfectly 

 into the front margin of AVL that without doubt it was attached to AVL and 

 had no connection with the lower jaw. This is clearly seen in Text-figures 63 and 64, 



which represent the sketch made by the 

 present author from Jaekel's original ma- 

 terial. Heintz mentioned IL in a description 

 of Acanthaspis (1929.1, .2). 



In Text figure 62 it is seen that IL in 



Text-figure 61' 



The ventral shield of Dimchthys intermedius 



Nwb., outside view. 



Text-figure 62. 

 The in tero- lateral plate: A, from outside; B, from inside. 



Im, lower margin; ov, overlapping margin ; ud, upper angle. 



