DEVONIAN FISHES OF IOWA 67 



On the basis of the foregoing distinction it follows that Pisces 

 proper are divisible into four subclasses, all of which were in 

 existence as early as the beginning of the Devonian, and have 

 followed distinct courses of development ever since. The an- 

 cestors of these subclasses are indeed unknown, but would prob- 

 ably fall under the designation of primitive Elasmobranchs.* 

 For determining the ordinal position of any fish within its appro- 

 priate subclass, the most satisfactory recourse is afforded by the 

 degree of specialization of the paired fins. It is now a well 

 established principle in palaeontology that both the median and 

 the paired fins of fishes originated in the first instance from con- 

 tinuous dermal folds, and during the course of their elaboration 

 passed through definite phases, the most important of which are 

 summarized by Smith Woodward in the following propositions : 



1. Fishes originally possesed (a) a continuous median der- 

 mal fold, and (b) a pair of continuous lateral folds, each sup- 

 ported by a regular series of parallel endoskeletal rods diverging 

 from the axial skeleton. 



2. These continuous folds (the median fold in most cases and 

 the lateral folds always) soon became subdivided, with a con- 

 comitant reduction in the size and number of their supports. 



3. Gradual and constant specialization has been marked by 

 the shortening-up of the endoskeletal supports of the resulting 

 fins, and by the concomitant strengthening of the dermal rays. 



4. In the course of this evolution the endoskeletal fin-supports 

 have eventually lost all direct connection and correlation with the 

 axial skeleton, those of the dorsal and anal median fins becoming 

 correlated instead with the dermal rays, each supporting one of 

 these rays. 



The order in which these stages in the evolution of paired fins 

 are successively passed through by the four leading divisions or 

 subclasses of fishes is apparent from the following table, which 

 we have reproduced after Smith Woodward : 



* See on this point President Jordan's remarks on C. T. Eegan's taxonomic 

 conclusions as presented in his paper on the "Phylogeny of Teleostomi". Guide 

 to the Study of Fishes, vol. 1, p. 623. 



