DEVONIAN FISHES OF IOWA 65 



sistent heterocercal tail; (3) completed balancing fins and homo- 

 cereal tail; and (4) a completed internal skeleton. 



Finally, to recapitulate the brief summary given by Woodward 

 in more general terms, "fossils prove that the earliest known 

 fish-like organisms strengthened their external armour so long 

 as they remained comparatively sedentary; that next the most 

 progressive members of the class began to acquire better powers 

 of locomotion, and concentrated all their growth-energy on the 

 elaboration of fins ; that, after the perfection of these organs, 

 the internal bony skeleton was completed at the sacrifice of outer 

 plates, because rapid movement necessitated a flexible body and 

 rendered external armour less useful; that, finally, in the highest 

 types the vertebrae and some of the fin-rays were reduced to a 

 fixed and practically invariable number for each family or genus, 

 while there was a remarkable development of spines. As sur- 

 vivors of most of these stages still exist, the changes in the soft 

 parts which accompanied the successive advances in the skeleton 

 can be inferred. Hence Palaeontology furnishes a sure basis for 

 a natural classification in complete accord with the development 

 of the group." * 



General Classificatory Scheme. — It is now in order to consider 

 the more salient features of classification adopted for arranging 

 the different groups of fishes in a natural system, one that is 

 founded on broad general distinctions, and aims to be expressive 

 of genetic relations. The fundamental principle which serves as 

 the basis of classification is that which takes cognizance of two 

 distinct types or plans of cranial structure among fishes, plans 

 that have been manifest at least ever since Lower Devonian 

 times, and between which no definitely intermediate conditions 

 are to be observed. Huxley has appropriately named one of 

 these types of cranial structure "autostylic", and the other 

 "hyostylic".t In the former, as illustrated by Chfmaeroids and 

 modern Lung-fishes, the upper segment of the mandibular arch 

 is directly fused with the chondrocranium, while the correspond- 

 ing segment of the hyoid arch is atrophied or absent. But in the 



* Woodward, A. S., The Relations of Palaeontology to Biology. Ann. Mag. Nat. 

 Hist. 1906, ser. 7, 18, p. 314. 



fHuxley, T. H., On Ceratodus forsteri, etc. Proc. ZooL.Soc. 1876, p. 40. 

 5 



