PALEOZOIC FISHES 227 



usually placed with diimceroids, will in all probability take a similar 

 position. 



The group of sharks has not yet furnished the all important key to 

 piscine descent. The evidence accumulates (Dean) that Cladoselachian 

 sharks stood near the base of the selachian pedigree; and the preserva- 

 tion of a number of their structures in histological detail shows that the 

 kidney extended far behind the region of the pelvic fins, making it 

 probable, therefore, that the cloaca was subcaudal and that in these 

 forms a portion of the gut w^as functional which today is retained in 

 shark embryos as post-anal. In the matter of the primitive fin struc- 

 tures of Cladoselachian sharks, we note that Jaekel ascribes pelvic 

 claspers to one of the specimens in the British Museum, but a renewed 

 examination of this slab convinces us that the evidence is unsubstantial. 

 The studies of the past decade have contributed additional ground for 

 believing that Acanthodian sharks were close relatives of Cladoselachids, 

 but more highly specialized (Dean). In some respects, it must be 

 admitted, they retain primitive structures, even more primitive than 

 known today in Cladoselachids. Thus, the curious jointing present 

 in the jaw (Jaekel) indicates a singularly close homology of jaw and 

 branchial arch. Eegarding Acanthodians, but in another line, Dollo 

 has made the interesting point that they were planktonivorous fishes; 

 and, solving of a minor puzzle, we have now the data that the Acan- 

 thodians included Gyracanthus (Traquair and Woodward) . Still another 

 smaller problem has been solved (Hay) in the case of Edestus; its curious 

 coil of teeth was placed in the s}Tiiphyseal region, as many believed 

 and as Eastman demonstrated indirectly — ^that is, on the evidence of 

 Campyloprion. 



Upon the origin of Dipnoi it is safe to say that no light has been cast 

 daring the past decade. 



Nor have the puzzles of Chimeroids been solved from the point in view 

 of paleontology. Evidence has indicated that Menaspis (Permian) is a 

 primitive Chimaeroid (Dean), which approaches very closely certain 

 Deltodont (primitive Cestraciont) sharks. It should here be mentioned 

 that Eegan maintains that Chimreroids were derived from still more 

 primitive sharks than early Cestracionts. In any event the findings of 

 paleontology as to selachian derivation receive confirmation on the side 

 of embryology, for the study of Chimseroids points out that their plan of 

 development is like that of sharks, but in numerous directions more 

 specialized. 



The past ten years have contributed no major documents to clear up 

 our knowledge of the early Teleostomes. In some cases, it is true, a 



