70 IOWA GEOLOGICAL SURVEY 



IV. 



Systematic Account of Devonian Fishes, with Special 

 Reference to those of Iowa and Adjoining States. 



"Wenn ich ein zerstreutes Gerippe finde, so kann ich es zusanimenlesen und 

 aufstellen; denn hier spricht die ewige Vernunft durch ein Analogen zu mir, und 

 wenn es ein Riesenfaulthiere ware." — Goethe. 



Class AGNATHA. 



Cyclostonies, or as they are sometimes called, the Marsipo- 

 branchs,in allusion to their pouch-like branchial sacs, include the 

 most primitive of all existing vertebrates. They are divided into 

 two orders, the first comprising the Hag-fishes or Myxinoids, and 

 the second the Lampreys. These orders are very distinct from 

 each other, though sharing important characters in common 

 which separate them widely from ordinary fishes. For instance, 

 they ^differ collectively from Pisces proper in the total absence 

 of limbs, together with pectoral and pelvic arches for their sup- 

 port, and, more strikingly still, in the lack of a lower jaw. Most 

 students are of the opinion that the absence of these structures 

 in Cyclostomes is a primitive character, although it has been 

 thought by some that jaws and limbs were primordially present, 

 and have since become lost through degeneration. As stated by 

 President D. S. Jordan, "there is no clear evidence that the 

 [sub-] class of Cyclostomes, as now known to us, has any great 

 antiquity, and its members may be degenerate offshoots from 

 types of greater complexity of structure." 



In regard to the occurrence of supposed extinct Cyclostomes, 

 such as the much debated Pakeospondylus, whose relations are 

 still considered doubtful, the same distinguished author speaks 

 as follows: 



