Acanthodu — Selarhii. 



85 



D, E. 



that some of the teeth named (Jladodiis also belong to this 

 order. 



Order III. — Acanthodii. 



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

 order of Elasraobi'anchs. They are small fishes exclusively No. 3, and 

 contined to Pala30zoic rocks, ranging from the Lower Devonian j^og d^^e^^' 

 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 Carbon Ifei-ous of 



Fig. 113. — AcaMkoOa K'cn'', Egert.; Coal-measuros, 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 Mesacanthns, 

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

 Devonian of Canada, is aluiost identical. ( 'liniatms, from tl<.e 

 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. Diplacanthns 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 chai-acterised, among other features, by the structure of 

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

 but never exhibit the branched an-angement seen in Pleuracan- 

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



