Acanthodu Selachii. 85 



that some of the teeth named Gladodus also belong to this 

 order. 



ORDER III. Acanthodii. 



The ACANTHODII form perhaps the most remarkable extinct Wall-case, 

 order of Elasmobranchs. They are small fishes exclusively No - 3 and 

 confined to Palaeozoic rocks, ranging from the Lower Devonian 

 to the Lower Permian. The tail is heterocercal, and each of 

 the fins, except the caudal, is armed in front with a formidable 

 spine, though in itself a mere membrane without cartilage 

 supports beyond the base. The eye is often surrounded by a 

 ring of plates, and the teeth when observable are fused with the 

 border of the jaws. There is no hard operculum, and the trunk 

 is covered with minute, quadrangular, shining scales; the 

 slime-canal of the " lateral line " passes between two series of 

 the latter. Acanthodes (Fig. 113), from the Carboniferous of 



FIG. 113. Acanthodes Wardi, Egcrt.; Coal-measures, Staffordshire. 



Scotland and England, and the Lower Permian of Germany, 

 is the typical genus, and is represented in the cases by numerous 

 nearly complete fishes. The beautifully preserved Mesacanthus, 

 from the Lower Old Red Sandstone of Scotland, and the Upper 

 Devonian of Canada, is almost identical. Qli/rnatius t from the 

 Lower Old Red Sandstone of Scotland, is remarkable for its 

 broad spines, of which three or four pairs are fixed along the 

 body between the pectoral and pelvic fins. Diplacanthus and 

 Parexus, of corresponding age, are also remarkable. 



ORDER IV. Selachii. 



The order of SELACHII, or sharks and rays proper, also 

 seems to have been represented in the Palaeozoic Era, though 

 there is no conclusive evidence of its existence before the Lias. 

 It is characterised, among other features, by the structure of 

 the paired fins, which are always supported by large cartilages, 

 but never exhibit the branched arrangement seen in Pleuracan- 

 thus, having either two or three basal pieces. The " Rays " are 



