GEOLOGICAL DISTEIBUTION. 195 



them, Onchus, still surviving from the Sihirian epoch. All 

 these spines are believed to be those of Choudropterygians, to 

 which order some pliiriciispid teeth (Cladodus) from the Old 

 Red Sandstone in the vicinity of St. Petersburg have been 

 referred likewise. 



The remains of the Ganoid fishes are in a much more 

 perfect state of preservation, so that it is even possible to 

 obtain a tolerably certain idea of the general appearance and 

 habits of some of them, especially of such as were provided 

 with hard carapaces, solid scales, and ordinary or bony fin- 

 rays. A certain proportion of them, as might have been 

 expected, remind us, with regard to external form, of Teleos- 

 teous fishes rather than of any of the few still existing Ganoid 

 types ; but it is contrary to all analogy and to all palseonto- 

 logical evidence to suppose that those fishes were, with regard 

 to their internal structure, more nearly allied to Teleosteans 

 than to Ganoids. If they were not true Ganoids, they may 

 be justly supposed to have had the essential characters of 

 Palseichthyes. Other forms exhibit even at that remote geo- 

 logical epoch so unmistakably the characteristics of existing 

 Ganoids, that no one can entertain any doubt with regard to 

 their place in the system. In none of these fishes is there 

 any trace of vertebral segmentation. 



The Palseichthyes of the Old Eed Sandstone, the syste- 

 matic position of which is still obscure, are the Ccphalaspidce 

 from the Lower Old Eed Sandstone of Great Britain and 

 Eastern Canada ; Ptcrichtliys, Coccosteus, and Dinichthys : 

 genera which have been combined in one group — Placodermi ; 

 and Acanthodcs and allied genera, which combined numerous 

 branchiostegals with chondropterygian spines and a shagreen- 

 like dermal covering. 



Among the other Devonian fishes (and they formed the 

 majority) two types may be recognised, both of which are 

 unmistakably Ganoids. The first approaches the stiU. living 



