238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



Arthrodira are divided into two separate parts : the head shield and 

 the body shield, which are movable and connected with each other 

 by a double joint, which permits a more or less free movement up or 

 down of the head in relation to the body. The head in Acanthaspis 

 is flat, the small eyes are situated laterally far forward in the skull. 

 Immediately in advance of them are the nostrils. 



The body shield in the Acanthaspids is rather low (fig. 8, A 2, 3 ; fig. 

 9, A), entirely flat below and somewhat arched above, with a sharp 

 bend along the median line. Particularly characteristic of the Acan- 

 thaspids, however, are the strongly developed immovable lateral 

 spines, which are located on both sides of the anterior part of the body 

 shield, behind the gill opening (fig. 8, A, B). If we view the an- 

 terior portion of the shield and the spines in cross-section it is not 

 difficult to understand that we are here dealing with a genuine fold of 

 the skin (fig, 8, A 4). It is therefore entirely natural that these 

 spines should be regarded as homologous with the pectoral fins of 

 other fishes. The spines are moreover not entirely horizontal, but 

 their anterior edge is somewhat lower than their posterior edge, that 

 is, they slant a little forward. 



The posterior part of the trunk and the tail in the Acanthaspids 

 was covered with rather solid scales as we can see quite distinctly 

 from an almost complete specimen of Acanthaspis found in the 

 German Lower Devonion and described by Broili (fig. 8, B). The 

 same specimen also shows us that the Acanthaspids were without 

 pelvic fins, and only had weakly developed medio-dorsal fins or 

 spines. It is particularly noticeable that the caudal fin also seems 

 to be lacking, as the body apparently simply tapers to a point. There 

 can be no doubt that the Acanthaspids were typical bottom forms 

 which swam very poorly. While swimming they used the posterior 

 part of their trunk and the tail as organs of propulsion. The organs 

 of equilibrium can be seen in their powerful spines and in their 

 dorsal fins. Special steering organs in the form of movable paired 

 fins are entirely absent. The spines probably not only served as 

 organs of equilibrium but also as gliding organs in a similar manner 

 as in the Cephalaspids and Pteraspids (figs, 2 and 7). Acanthaspis 

 would rise from the bottom by powerful strokes of its tail, descend- 

 ing again more slowly, gliding on its spines. 



The Acanthaspids disapi>ear completely in the Middle Devonian, 

 Avhere they are replaced by a great number of other Arthrodirans 

 showing the most diverse adaptations. Some adapted themselves 

 more and more completely to a bottom mode of life, developing a 

 rather flat head and body shield. Others, on the other hand, be- 

 came free swimmers. A veiy well-known form from the Middle 

 Devonian, Coccosteus (fig. 8, E; fig. 9, B), is fairly close to the 

 Acanthaspids, but shows distinct adaptations to more efficient swim- 



