direct derivative of a primitive palaeoniscoid type, whereas 

 Amia appears to be a highly modified actinopterygian more 

 closely related to the teleost. 



In some ways, as in its scale structure, Amia appears to 

 be as advanced as the teleost, but in other features it is 

 more primitive, as in the less ossified endocranium, the 

 retention of supramaxillae, and in the heavier bones through- 

 out the body. Amia agrees with the teleost in having an 

 interopercle and apparently is a derivative of one of the 

 early lines in which this bone appeared. The interopercle 

 may be a parallel development in several lines or it may 

 indicate common ancestry, having been developed only 

 once, early in the evolution of the actinopterygians. 



The evolution of the choanates suggests that primitively 

 the lungs were highly developed, aerial respiration was well 

 established, and a three-chambered heart was present. This 

 line underwent a period of radiation which produced several 

 kinds of fishes and the ancestral tetrapod. The tetrapods 



have been placed between the osteolepids and the dipnoans 

 to indicate that structurally they are somewhere in between 

 these two groups. The tetrapod line is not identified as am- 

 phibian for the simple reason that when it first appeared, 

 it was already a complex array of types, rather than a single 

 simple line. Furthermore, the pelycosaur and sauropsid rep- 

 tiles appeared almost as early in the record as the amphib- 

 ians. The Amphibia then may be only an array of types 

 paralleling in some respects the two early lines of reptiles. 

 As the two lines of reptiles developed, one gave rise to the 

 mammals and the other to the living reptiles and birds. 



These superficial remarks leave many possibilities for in- 

 terpretation and documentation to the student, who may 

 evaluate them and form his own conclusions. This review of 

 phylogenetic concepts and facts should at least strengthen 

 the impression that the classifications of vertebrates are open 

 to refinement and that there is much yet to be learned about 

 the chordates and their evolutionary history. 



448 • A SUMMATION OF CHORDATE MORPHOLOGY 



