NORTH STAFFORDSHIRE COAL FOSSILS. 213 



and teeth found in the same strata is exceedingly limited. 

 " The mere discovery of teeth and spines in the same strata 

 cannot be relied upon as evidence of their relationship. 

 The only evidence of any value is the discovery of the 

 conjunction of teeth and spines in one and the same indi- 

 vidual."* Recent discoveries show that Helodus and 

 Pleurodus, genera not uncommon in the sub-Carboniferous 

 Limestone, had spines on their dorsal surface which have 

 been found in situ in conjunction with the teeth. Ctena- 

 canthus has also been found both in this coal-field and in 

 the Scotch coal-measures in conjunction with teeth which 

 were previously known as a distinct species allied to the 

 genus Cladodus. These discoveries are of great importance, 

 as they tend to lessen the already long list of species, 

 many of which no doubt have been founded on insufficient 

 data. 



A considerable amount of investigation must still be 

 made before a more comprehensive list can be given of the 

 Plagiostomous fishes which have been found in this coal- 

 field. No class of fishes occurs in such a condition so 

 fragmentary, and hone is more difficult to study than this. 

 The following list will be found to contain the whole of 

 the genera and species of this order which up to the present 

 time have been recognized. When, however, any additional 

 species have been identified a further enumeration will be 

 given to supplement the existing list. 



The Ganoid order, so called from the bright enamelled 

 surface of their dermal covering, is represented in this 

 coal-field by about twenty genera and upwards of thirty 

 species. Fishes of this order differ from those of the Placoid 

 order by having the body encased in a strong covering of 

 thick bony scales, of which, amongst existing Ganoids, the 



* Quar. Jour. Geol. Soc., vol. ix., p. 281. 



