PLEUROPTERYGII. 29 



In the Devonian strata remains of Elasmobranch dermal 

 armour are more numerous, and fine specimens of members of 

 the singular Order of Acanthodii are abundant. The latter 

 admit of systematic description and are noticed below (p. 35) ; 

 but the former only suffice to indicate that among the earliest 

 Elasmobranchs there was o*ne great group characterized by a 

 remarkable development of the dermal armour into overlapping 

 plates a group which survived throughout the Carboniferous 

 period antf may even have ranged into the Permian. 



The commonest of the Devonian fossils belonging to these 

 unknown armoured sharks are named Psammosteus, and the 

 plates are evidently formed by the fusion and thickening at the 

 base of clusters of simple tubercles like those scattered through 

 the Upper Silurian rocks. Some of the plates are extensive 

 aud ornamented only on one side, with broad smooth edges 

 which were overlapped either by skin or by adjoining plates. 

 Others are triangular or acuminate, sometimes solid, sometimes 

 partly hollow, and ornamented on both sides ; but in these the 

 ornament usually extends further down one side than the other, 

 and they may thus be compared with the paired spines of such 

 Acanthodians as Climatius and Pareocus. 



The various portions of dermal armour known as Oracanthus, 

 from the Lower Carboniferous, also probably belong to a fish 

 closely resembling Psammosteus ; and an Elasmobranch only a 

 little less armoured, ranging throughout the Carboniferous, is 

 Gyracanthus, represented by powerful paired fin-spines and by 

 more or less conical and triangular paired spines which are 

 almost destitute of ornament. 



ORDER 1. PLEUROPTERYGII. 



Judged by the condition of the paired fins, the most primi- 

 tive type of Elasmobranch hitherto discovered is the Upper 

 Devonian (or Lower Carboniferous) genus Cladoselache. It is 

 the only known representative of the order PLEUROPTERYGII, 

 which exhibit the nearest approach to the hypothetical lateral 

 fin-fold yet met with. The small basal cartilages of the paired 



