180 



Bashford Dean 'Memorial Volume 



Arthrodire carapace and thought that this joint was placed between two parts of 

 the body carapace. The next year (1857) Pander gave a relatively detailed description of 

 it in Coccosteus, Heterostius and Homostius, with many drawings. 



In later papers of well'known in- 

 vestigators of the Arthrodira (Traquair, 

 Woodward, von Koenen, Newberry, 

 Dean, Eastman, Hussakof), surprisingly 

 little attention has been given to the 

 structure of this part of the Arthrodiran 

 carapace. This is especially notable 

 when we remember that the name 

 "Arthrodira," proposed by Woodward 

 (1891.1), was based on this character. 

 Not until 1919 was a detailed descrip- 

 tion of this joint published. This was 

 by Jaekel who pointed out its great im- 

 portance in an understanding of the 

 structure of the Arthrodira. In the 

 same year (1919) Adams also called at- 

 tention to this unusual structure, and 

 lastly Abel (1928) described it in detail 

 in Heterostius from Esthonia. 



In Dinichthys, the joint between 

 the head and body has never been care- 

 fully studied.'" All authors have been 

 content to state the fact that the joint 

 socket is to be found on EB, and that the 

 condyle is developed on ADL, without 

 describing their structure or trying to 

 explain how they work. As a matter 

 of fact, this structure is the most characteristic one in the Arthrodira, and, except in the 

 Antiarcha (Asterolepida), is unknown in any other vertebrate. Its strong development 

 in all Arthrodira shows clearly its great importance to the animal and gives us at least a 

 possibility of understanding this mechanism. I shall now attempt to give an accurate 

 description of the connection between the head and body carapace. 



As we know, on the hind margin of EB is a deep, long joint socket (Text-figure 70 

 A, B, C; fg). After Jaekel (1919), we call it the "fossa glenoidalis." In section it is 

 semicircular (B C); broader in front, it narrows insignificantly backward. The upper 

 margin of £B runs in a solid, slightly curved process, bordering the joint socket from above. 

 We will call it the upper joint process (ujp). On the underside, the fossa glenoidalis is 



Text-figure 68 (large). 

 A reconstruction of the body carapace in Dinichthys seen 

 from the front. 

 kd, condyle; Sfi., the short spinal; w, hind wing of AL. 



Text-figure 69 (small). 



The body carapace of an Acanthasfiida from Spitsbergen 



(Heintz) in front view. 



Sp., the much elongated spinal plate. 



' The most complete description was made by Kemna (1904). 



