In Silurian and Devonian Epochs 



121 



that the evolution of fishes up to this stage doubtless con- 

 sisted in the appearance of groups of soft-bodied scaleless 

 animals with cyclostome affinities, of which the remarkable 

 Palaeospondylus gunni (Fig. lo) from the Old Red rocks 



of Caithness is the only known 

 illustration. Alongside such 

 there evolved — probably from 

 a Thelodus-like ancestry — the 

 sluggish, toothless, and heavily- 

 armored bottom-feeders, that 

 have been included in the great 

 group Ostracodermi. These 

 reached their climax — alike as 

 to structure and abundance of 

 Individuals — in the late Silurian 

 times, and were becoming rare 

 or extinct in early Old Red age. 

 But meanwhile, evolving from 

 a Birkenia-Lasanius ancestry of 

 Silurian age, the more lithe 

 forms that were ancestral, in the 

 writer's estimate, to the ganoids 

 and elasmobranchs, were in- 

 creasingly asserting themselves, 

 and became dominant in fresh- 

 waters of the Lower Old Red 

 period. So it should be special- 

 ly emphasized that the fore- 

 runners of all the later Selachi- 

 ans were of freshwater origin. 

 On the European continent, 

 however, an apparent exception 

 to the above has greatly puzzled 

 the writer. For Schliiter and Traquair {77: 723) have de- 

 scribed the heterostracous genera Drepanapis and Phlyctae- 

 naspis from the Hunsriicken slates of Gemiinden. These 

 genera are asserted to be in direct association with crinoids, 

 starfishes, trilobltes, bivalve and cephalopod molluscs. This 

 causes Traquair to declare : "It is perfectly clear from the 

 contained Invertebrate fossils that the Hunsriick slates are 



Fig. 10. Restored figure 

 of skeleton, Palaeospondylus 

 gunni. c, calcified cephalic 

 cirri; pa, auditory capsule; 

 tp, supposed nasal capsule; 

 X, postoccipital plate. (After 

 Traquair.) 



