444 The Evolution of Fishes 



lamprey fossil of the Devonian of Scotland, Pakeospondylus, 



has little in common with the true lampreys. 



The Ostracophores. Besides the lampreys the Devonian seas 

 swarmed with mysterious creatures covered with an armor of 

 plate, fish-like in some regards, but limbless, without true jaws 

 and very different from the true fishes of to-day. These are 

 called Ostracophori, and some have regarded them as mailed 

 lampreys, but they are more likely to be a degenerate or eccen- 

 tric offshoot from the sharks, as highly modified or specialized 

 lampreys, a side offshoot which has left no descendants among 

 recent forms. Recently Professor Patten has insisted that the 

 resemblance of their head-plates to those of the horseshoe crab 

 (Limulus] is indicative of real affinity. 



Among these forms in mail-armor are some in which the 

 jointed and movable angles of the head suggest the pectoral 

 spines of some catfishes. But in spite of its resemblance to a 

 fin, the spine in Pterichthyodes is an outgrowth of the ossified 

 skin and has no more homology with the spines of fishes than 

 the mailed plates have with the bones of a fish's cranium. In 

 none of these fishes has any trace of an internal skeleton been 



FIG. 254. An Ostracophore, Cephalasjns lyelli Agassiz, restored. Devonian. 

 (After Agassiz, per Dean.) 



found. It must have retained its primitive gelatinous character. 

 There are, however, some traces of eyes, and the mucous channels 

 of the lateral line indicate that these creatures possessed some 

 other special senses. 



Whatever the Ostracophores may be, they should not be in- 

 cluded within the much-abused term Ganoidei, a word which was 

 once used in the widest fashion for all sorts of mailed fishes, but 

 little by little restricted to the hard-scaled relatives and ances- 

 tors of the garpike of to-day. 



The Arthrodires. Dimly seen in the vast darkness of Paleo- 

 zoic time are the huge creatures known as Arthrodires. These 

 are mailed and helmeted fishes, limbless so far as we know, 



