The Primitive Fishes 271 



occurred freshwater connections, either as lakes, marshes, 

 or rivers, which may frequently have changed their depths, 

 configuration, and relations, during successive periods of 

 time, but which so established a linked-up connection 

 that species or genera of the group occupied some part of 

 the above extensive area between the period of the Lower 

 Old Red and that of the Permian. 



The highly suggestive observations of A. S. Woodward 

 on the evolutionary relation of the paired fins in Acanthodes 

 (igg:^) should here be noted. 



The two groups of the Cladoselachii (or Pleuropter- 

 gyii) and the Ichthyotomi include elasmobranchs that show 

 many afi'inities with the Acanthodii Both, however, so far 

 as now known, appear later in the earth's history. For 

 the best known representative of the first — Cladoselache 

 fyleri — as well as three more recentlv described species, 

 are only reported from Upper Devonian — or as Newberry 

 viewed them Lower Carboniferous — shales of Ohio. An- 

 other and allied species has been found in the Calciferous 

 beds of Kilbride in Scotland, while teeth like those from 

 Ohio, have been found in North America, Europe, and even 

 India. The exact beds, from which all of these were de- 

 rived, clearly indicate a freshwater environment. 



The highly interesting but imperfectly known Chondren- 

 chelys problematica of Traquair, that as yet is known only 

 from the Calciferous rocks of Eskdale Scotland, seems in 

 some structural points to connect the Pleuropterygii and 

 the Ichthyotomi. 



The group Ichthyotomi is of special interest, alike 

 from the standpoints of structure, distribution and phylo- 

 geny. For structurally of Pleiiracanthus (Fig. 44) it has 

 well been said that "it is a form of fish which might with 

 very little modification become a selachian, dipnoan or 

 crossopterygian." (200:32) Thus the notochord is a con- 

 tinuous tube with slight calcifications, the cartilaginous endo- 

 skeleton has only small granular calcifications, the paired 

 fins consist of a median segmented axis bearing fringe-like 

 lateral cartilages, and the exoskeleton consists of small 

 placoid granules, in addition to a strong median dorso- 

 occipital spine. 



