EARLY ARMORED FISHES 



165 



Fig. 46. The Ostracoderm Palceaspis 

 OF Claypole as Restored by Dean. 



the law of convergence among the aquatic amphibia, reptiles, 

 and mammals as one of the invariable effects of the coordina- 

 tion of the mechanism of locomotion with that of offense and 

 defense. In each of these four or five great radiations of body 

 form, from the swift-moving 

 to the bottom- or ground- 

 living, slow, armored types, 

 there is usually an increase of 

 bodily size, also an increase of 



specialization, the maximum in both being reached just before 

 the period of extinction arrives. 



Early Armored Fishes 



The armored Ordovician ostracoderms are very little 

 known. The Upper Silurian ostracoderms enjoyed a wide 



distribution in Europe and 

 America. They include 

 both the fusiform, free-swim- 

 ming type (Birkenia) and 

 the broadly depressed ray- 

 like types {Lanark ia, etc.). 

 Apparently they had not 

 yet acquired cartilaginous 

 lower jaws and were in a 

 lower stage of evolution than 

 the true fishes. 



The armature is from 

 the first arranged in shield 

 and plate form, as seen in 

 Palceaspis, from the Upper 

 Silurian Salina time of Schu- 

 chert. In this epoch we 



Fig. 47. The Antiarchi. 



Armored, bottom-living Ostracoderm type, Bo- 

 ///r/o/f/^w, from the Upper Devonian of Canada, 

 with chitinous armature and a pair of anterior 

 appendages analogous to those of the euryp- 

 terid crustaceans. This cluster of animals was 

 undoubtedly buried simultaneously while 

 headed against the current in search of food 

 or for purposes of respiration. After Patten. 



