CHAPTER XXXII 

 THE CLASS OSTRACOPHORI * 



STRACOPHORES. Among the earliest vertebrates act- 

 ually recognized as fossils belongs the group known as 

 Ostracophori (oarpa^os, a box ; 0ope&?, to bear) . These 

 are most extraordinary creatures, jawless, apparently limb- 

 less, and enveloped in most cases anteriorly in a coat of mail. 

 In typical forms the head is very broad, bony, and horseshoe- 

 shaped, attached to a slender body, often scaly, with small 

 fins and ending in a heterocercal tail. What the mouth was 

 like can only be guessed, but no trace of jaws has yet been 

 found in connection with it. The most remarkable distinctive 

 character is found in the absence of jaws and limbs in connec- 

 tion with the bony armature. The latter is, however, sometimes 

 obsolete. The back-bone, as usual in primitive fishes, is de- 

 veloped as a persistent notochord imperfectly segmented. The 

 entire absence of jaw structures, as well as the character of the 

 armature, at once separates them widely from the mailed Arthro- 

 dires of a later period. But it is by no means certain that 

 these structures were not represented by soft cartilage, of which 

 no traces have been preserved in the specimens known. 



* This group was first called by Cope Ostracodermi a name preoccupied 

 for the group of bony trunkfishes, Ostracidce. The still earlier name of 

 Placodermi, chosen by McCoy (1848), was intended to include Arthrodires as 

 well as Ostracophores. Rohon (1892) calls the group Proiocephali, and to 

 the two orders he assigns the names Aspidorhini and Aspidocephali. These 

 groups correspond to Heterostraci and Osteostraci of Woodward. Another 

 name of early date is that of Aspidoganoidei, given by Professor Gill in 1876, 

 but not defined until 1896. These fishes are, however, not "Ganoids" and 

 the name Ostracophori seems to receive general preference. The group Pel- 

 tacephalata of Patten corresponds essentially to Ostracophori, as does also the 

 order Hypostomata of Gadow. 



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