FOSSIL FISHES. 



19 



strong defensive spines in front of the paired and median fins, but 

 not in front of the caudal fin. They have minute rhombic 

 enamelled scales, ornamented in some species by fine striae. From 

 the similarity of form of the dorsal spines and their mode of 

 implantation ; the small and shagreen-like scales, which scarcely 



Fig. 25,—Diplacanthus striatus, Old Red, .'Cromarty. <n. First dorsal fin; d2, second 

 dorsal fin; p, pectoral fin; c, ventral; a, anal fin; c, caudal fin. Each fin has a strong 

 defensive spine in front. 



overlap, the cartilaginous skeleton, and naked branchial arches, 

 they are considered by some ichthyologists to occupy a position 

 intermediate between the Plagiostomata and the Ganoidei (Table- 

 case 34). 



Fig. 2i.—Acanthodes, sp. (restored figure, after Kner and Roemer), Permian. 



They are represented by Diplacantlius (Fig. 25), CJieiracanthiis, 

 Climathis, Euthacanthus, and JParexus, from the Old Eed Sandstone, 

 and Acanth odes (Fig. 24), ranging from the Old Eed to the Permian. 



Sub-order II. — Placodei;ma.ta. 



The Placodermata (Wall-case 4) are characteristic of the Old 

 Eed Sandstone or Devonian rocks. The head and anterior portion 

 of the body were covered with large bony plates, sculptured and 

 ornamented with beads of enamel. 



c 2 



