Acanthodians. 45 



Dipterians, or rather, whether it indicate, by its simple fins, 

 the first degree of approach to the type of the Sauroides pro- 

 perly so called. In the latter case, we should have, in the 

 Sauroides of the old red sandstone, a gradation similar to 

 that which exists in the Cephalaspides. 



The Acanthodians embrace in their history only two for- 

 mations, the old red sandstone and the coal measures ; more 

 recent formations furnish no traces of them. This also is a 

 very particular type, in no manner connected with the other 

 families of the Ganoides. It is true that the form of the body 

 does not deviate from those with which we are familiar, but 

 the manner in which their bodies are covered certainly pre- 

 sents a very decided character. Those small rhomboidal 

 scales, scarcely visible, which make the skin look like sha- 

 green, have nothing like them in the whole class of fishes ; 

 for the shagreen of the Plagiostomes is formed of entirely dif- 

 ferent elements. It may be remarked that in general the 

 anomalous types, which deviate most from the normal types, 

 are also of very brief duration, and continue only during one 

 or two epochs of the history of the earth, after which they 

 terminate, without our remarking afterwards the types which 

 may be regarded as those that have replaced them. This is 

 likewise the case with the Cephalaspides. It is the same 

 with the Acanthodians. In the fusiform Ganoides of more 

 recent epochs we find neither scales in the form of shagreen, 

 nor large spines, in the form of prickles, which stand erect 

 upon the fins. This type becomes entirely extinct with the 

 coal formation. 



Of all the Ganoides of the old red sandstone, the Celacan- 

 thes are the only ones which have a lengthened history ; for 

 they continue as far as the chalk formations, where they ter- 

 minate in the genus Macropoma. I have already shewn, in 

 treating of this family, what difficulties we have to encounter 

 when we wish to limit it rigorously, and assign to it definite 

 characters, and how probable it is that it will ultimately be 

 divided into many distinct families. But, apart from these 

 considerations, which are not yet founded on facts sufficient- 

 ly numerous, it is certainly in the old red sandstone that the 

 family of the Celacanthes acquires the most considerable de- 



