DEVONIAN FISHES OF IOWA 185 



other writers, that further description of it here would be super- 

 fluous. It furnishes a most valuable standard of comparison for 

 referring the detached plates of other Arthrodires to their nat- 

 ural position, and for correlating the numerous minor variations 

 observed in different members of the group. Interest centers 

 in it also from the fact that this is commonly looked upon as the 

 progenitor of the remarkable series of Dinichthyids that con- 

 stitute perhaps the most striking feature of the late American 

 Devonian fish fauna. That it is itself descended from a more 

 primitive Arthrodiran ancestor having a simpler cranial pat- 

 tern and a triturating type of dentition, seems to be an inevi- 

 table conclusion in the light of all the known facts. A weighty 

 argument in favor of the Dipnoan origin of Arthrodires would 

 lie in the confirmation of Jaekel's reported discovery of the 

 long sought articulare in Coccostean mandibles. 



Neural and haemal arches are occasionally seen in well pre- 

 served skeletons, but never any traces of axial segmentation; 

 nor do ossified ribs occur in any member of this family. The 

 structures which have sometimes been mistaken for ribs are in 

 reality Jiaemal arches.* There is one dorsal fin, the tail tapers 

 gradually and to all appearances was diphycercal, but definite 

 information on this point and concerning the presence of an anal 

 fin is lacking. Indistinct traces of pelvic fins have been observed 

 in Coccosteus, and an arch for their support seems to be repre- 

 sented by a pair of club-shaped internal plates, which Jaekel 

 somewhat inappropriately calls "ilia". A pair of short, deep 

 external plates lying immediately in front of the ventral armor 

 in Coccosteus, but not represented in Dinichthys so far as 

 known, has been thought by some to indicate the presence of 

 a pectoral arch. The fact remains, however, that even the most 

 exquisitely preserved specimens have failed thus far to reveal 

 the slightest traces of pectoral fins, and the so-called pectoral 

 spine or "Ruderorgan" in Coccosteus and Brachydirus, which 

 is apparently fused in some species with one of the dermal plates 

 protecting the sides, can in no sense be homolosdzed with a pec- 



* Jaekel, O., Ueber Coccosteus und die Beurtheilung der Placodermen. Sit- 

 zungsber. Ges. Naturforsch. Freunde, 1902, p. 107, restoration. 



