xxxiii 



? ACANTHODEI. 

 SUPERORDER ClIONDROGANOIDEI. 



(Aberrant.) 

 Order Chondrostei. 

 Order Glaniostomi. 



SUPERORDER BRACHIOGANOIDEI. 

 (Leading to the Dipnoans.) 

 Order Actinistia. 

 Order Crossopterygia. 



SUPERORDER DIPNOI. 



(Leading to the Batrachians.) 



Order Sirenoidei. 



SUPERORDER HYOGANOIDEI. 

 (Leading to the Teleosts.) 

 Order Rhomboganoidei. 

 Order Cycloganoidei. 



ON THE TERMS &quot; HIGH&quot; AND &quot; Low.&quot; 



The conclusions resulting from the study of the preceding types may 

 render advisable the reconsideration of the reasons of the discrepancy 

 existing among naturalists as to the sequence of the several forms referred 

 to. It has been remarked (p. xx) that the reasons were obvious, and the 

 discrepancies are undoubtedly (1) in part the results of the appreciation 

 of certain truths, and their exaggeration at the expense of others, and to 

 the neglect of the consequences flowing from that cause, and (2) partly of 

 psychological prejudices. 



It is a well-assured truth that the Dipnoans are the fishes most nearly 

 related to the Batrachians, and consequently, if nothing else were to be 

 considered, they should undoubtedly be placed next to them. But if this, 

 per se, would be a satisfactory procedure, the problem then arises, what 

 shall be done with the other forms ? If the Dipnoans are at one extreme 

 and the Leptocardians at the other, between them must necessarily inter 

 vene the typical fishes as well as the true Ganoids and the Elasmobran- 

 chiates. And if, now, the question of the relative position of the Dipnoans 

 be properly settled, the equally important one and more vital one on ac 

 count of the numbers involved recurs, are we any nearer the truth in ap 

 proximating next to the Dipnoans, the Elasmobranchiates, the Ganoids, 

 and finally the Teleosts, which last will be next to the Marsipobranchiates ? 

 October, 1872. 3 



