966 CLASS PISCES. 



placed in it. The Acanthodeans appear to be in some respects 

 intermediate between the Ganoids and Elasmobranchs, and should 

 perhaps constitute an order by themselves. They have the body, 

 which is more or less elongated and compressed, covered with 

 shagreen-like scales, and with the lateral line running between two 

 rows of such scales. The tail is heterocercal ; and the fins have 

 strong spines, which, except in the pectorals, are merely inserted 

 between the muscles. There is considerable doubt as to the pres- 

 ence of cranial bones or of a gill-cover ; but there is a ring of bones 

 round the orbit. The vertebral column is cartilaginous, and teeth 

 are either wanting or are very minute and sharp. 



In their cartilaginous skeleton, the not improbable absence of an 

 operculum, the structure of the scales and position of the lateral 

 line, as well as in the spines of the median fins, the Acanthodea 

 approach the Elasmobranchei ; but the articulation of the pectoral 

 fin-spine to the pectoral girdle is a character of the Teleostean Si- 

 Iwidce, while the orbital ring is a character of the higher Ganoids like 

 the Palceoniscid(£. 



Family Acanthodid^e. — All the genera may be provisionally 



Fig. 8gS.—Acant/iodes ; from the Permian of Europe. (After Kner and Roemer.) 



included in a single family, of which the type genus Acanthodes (fig. 

 898), as now restricted, ranges from the Carboniferous to the Per- 

 mian of Britain and the Continent. The head is very short and 

 blunt, and there is but a single dorsal fin placed immediately above 

 the anal, while it is thought that teeth were absent. Mesacanthus, 

 of the Scottish Devonian, includes small fishes distinguished from 

 the last genus by the presence of an intermediate pair of small 

 spines between the pectoral and pelvic fins ; it is represented in 

 the Devonian of Canada. Closely allied is Acanthodofisis, from 

 the Carboniferous of Northumberland, in which there were numer- 

 ous minute teeth; while Chiracanthus (fig. 899, 1), of the Scottish 

 Devonian, is distinguished by the dorsal fin being placed in advance 

 of the anal. In Diplacanthus (fig. 900) there are two dorsal fins, 

 of which the second is placed above the anal ; each pectoral fin 

 has two spines ; while there are minute spines between the pectoral 



