60 



IOWA GEOLOGICAL SURVEY 



along the entire length of the abdomen. Therefore, if the sep- 

 arate median fins were produced by the subdivision of a primi- 

 tively continuous median membrane along the back and the 

 lower side of the tail, the paired fins arose similarly by the sub- 

 division of continuous membranes which extended as a sym- 

 metrical pair along the outside walls of the body-cavity. 



l.SJl. 



Fie "5 Outlines of Acanthodian Fishes, illustrating their gradual elongation in 

 shaoe and loss of intermediate spines during successive periods. A, Climatius scutiger 

 Feert Lower Old Red Sandstone; Scotland. B, Mesacanthus mitchelh (Egert.); ibid. C, 

 Acant'hodes sulcatus As. Lower Carboniferous; Edinburgh. D, Acanthodes gracilis Roemer. 

 Tower Permian- Bohemia, a, anal fin; d, dorsal fin; i. up., ' intermediate spines";;), pair 

 of pectoral fins; v, pair of pelvic fins. (From Smith Woodward, partly after Traquair and 

 Fritsch.) 



