112 VERTEBRATE ZOOLOGY 



Sub-Order 13. Plectognaihi (7 families, including file-fishes, 



trunk-fishes, puffers, porcupine fish, 

 and sun-fish). 

 SUB-CLASS III. DIPNEUSTI (Dipnoi) Lung-Fishes. 



X2 extinct and 2 living families, includ- 



Johh bo^| [ ing Neoceratodus, Protoperus, and 



Lepidosiren). 



APPENDIX (TO TRUE FISHES) 



I. PAL,EOSPONDYLn>,E (1 family, between cyclostomes and fishes). 

 II. OSTRACODERMI . (3 orders of 8 families, mostly armored 



fishes). 



III. ANTIARCHI (1 family of mailed fishes). - 



IV. ARTHRODIRA (1 family of mailed fishes). 3^ 



"' ' : ' ', 

 It will be of interest to note that of the 226 families of true fishes 



listed in the above classification, ~i7 2 belong to the order Teleostei. 

 There are 32 families of Elasmobranchii, 9 of which are extinct. The 

 remaining 22 families are divided among the ganoids and dipnoans, 

 Although the Teleostei are unquestionably the characteristic 

 fishes of the present and may be considered the dominant creatures of 

 the waters of modern times, the elasmobranchs, represented most 

 typically by the sharks, are still, as they have been since Devonian 

 times, a powerful, predaceous tribe, that has ruled the waters through 

 strength and savagery; though not characterized by such sheer num- 

 bers nor by so many specializations and adaptations as are the teleosts. 



SUB-CLASS I. EL/SMOBRANCHII 



The present day sharks, though not as primitive as the extinct 

 types of elasmobranchs, are relatively a conservative group. Though 

 they have come through millions of years of evolution some of the 

 sharks (Fig. 67) notably the Notidanidae and the dog-sharks and rela- 

 tives, are still very generalized aquatic vertebrates and serve well 

 to illustrate primitive vertebrate morphology. On that account they 

 are used extensively as a main-line vertebrate type (stem-type) in 

 courses in comparative anatomy. Adaptive radiation has taken 

 place among the elasmobranchs less than in other groups. For the 

 most part they have retained the elongated spindle-shaped body, the 

 lightly armored integument, and active predaceous habits, character- 



