The Structure of Dinichthys 



179 



In Text-figure 66A is given a picture of the lower part of AL with the attached Sp 

 and IL. Here it is not difficult to recognize Newberry's clavicular. Text-figure 66B gives 

 the clavicular from above showing where the single plates were connected. By the help 

 of IL and Sp, the dorsal shield was joined to the ventral shield. The lower thick margin" of 

 IL was joined to the front margin of A VL. Its curved, hind part fits into the curved part 

 on the side of the front margin of AVL. The side process on AVL touches the hind 

 margin of Sp. As in many other Arthrodira, AL itself does not come in contact 

 with AVL. 



It only remains to take a 

 look at the complete reconstruc- 

 tion of the Dinichthys body cara- 

 pace, shown in Text-figure 67 

 in side view, and on Text-figure 

 68 from the front. Here it is 

 seen that the Dinichthys body 

 carapace forms a relatively nar- 

 row ring, especially narrow in 

 the contact points between the 

 ventral and dorsal shields, where 

 it is consolidated with the hind 

 wing (Text-figures 67 and 68 w) 

 of AL. All the plates fit together 

 very well, and as a whole the 

 carapace makes a very solid struc- 

 ture, serving not only as armor to 

 protect the body, but also, in no less degree, as an attachment for the strong head 

 and body muscles. 



The front view (Text-figure 68) shows us that Dinichthys was in reality narrower and 

 higher than formerly supposed. It is interesting to note how much, seen from this view, 

 the carapace of Dinichthys is like that of Acanthaspida from Spitsbergen (Heintz, 1929.1, 

 .2), one of the oldest known Arthrodira. The position of IL and its contact with AL and 

 AVL is identical. On the other hand, the long, strongly developed spine of Acanthaspida 

 (Text-figure 69) is reduced in Dinichthys to only a short plate. 



THE CONNECTION BETWEEN THE HEAD AND THE BODY CARAPACE 

 It is now necessary to describe how the head and body carapace are joined. This is one 

 of the most crucial matters in our study of Dinichthys, and calls for careful consideration. 



THE JOINT BETWEEN THE HEAD AND THE BODY 



From the earlier investigations we know that the head shield and the body carapace ot 

 the Arthrodira were connected by means of a movable joint. The first worker to describe 

 this structure was Assmuss (1856). However, he misunderstood this structure of the 



