The Primitive Fishes 265 



be accepted now that the fin-spines named Cheir acanthus, 

 Diplacanthus, Eiithacanthus, Gyracanthus, and Macracan- 

 thus belonged to acanthodean forms like Acanthodes or 

 Climatius; that Triodus, Diplodiis, Didymodiis, Thrinaco- 

 diis, and probably Cladodiis, are isolated teeth, while iso- 

 lated spines like those called Comps acanthus, Orthacanthiis, 

 and Xenacanthus are defensive parts belonging to pleura- 

 canthid types (795:2). 



The genus Acanthodes (Fig. 9e, p. 119) or Acanthoessiis 

 appears first in Lower Old Red rocks of Northern and 

 Central Scotland, in which also the allied genera Climatius 

 and Parexus are found. In the same or in related rocks 

 Diplacanthus, Ischnacanthus and Cheiracanthus spines are 

 often abundant, and are isolated parts of the three above 

 named genera. According to present knowledge they were 

 very abundant in lakes of Old Red age, that occupied areas 

 over N. Scotland. Thence they seem to have spread, with 

 lapse of time, westward into Canadian lakes that covered 

 in part the Scaumenac region, as well as eastward into 

 Russia. So their remains are encountered in the upper 

 freshwater Devonian strata of these widely apart regions. 



But the genus Acanthodes evidently persisted near or in 

 its ancestral centre, while the others died out largely or 

 wholly at the close of the Devonian. The result is that in 

 freshwater Calciferous rocks of Dumfriesshire, in Scotland, 

 and upward through the Coal Measures of Scotland and 

 England, the genus persisted; later in Permian rocks of 

 France, Germany and Bohemia it remained fairly abundant, 

 but then gradually disappeared. 



The above geologic distribution again suggests geo- 

 graphic and biologic continuity from W. Russia through 

 Britain to Canada at least, and throughout in freshwater 

 regions. Defence rather than offence is also suggested 

 for the entire series. For the lithe shape of body, the well- 

 developed fins, the powerful defensive fin-spines, and the 

 small closely set placoid scales, all conform to the former, 

 while the entire absence or small size of teeth except in 

 Acanthodopsis and Ischnacantlius suggest weak or inoffen- 

 sive relations. And wherever fossilized alongside other 

 organisms, the latter invariably proclaim a lake, swamp, 



