XVII ELASMOBRANCHII-— ACANTHODEI 441 



analogous to clavicles or cleithra and infra- clavicles can be 

 recognised. Each pectoral spine forms the preaxial margin of 

 the fin, and behind it there is a series of ceratotrichia. Nothing 

 is known of the endoskeletal supports, but having regard to the 

 nature and proportions of the pectoral spines it may be inferred 

 that the exoskeletal elements of the fins predominate over the 

 former to an extent which is only paralleled elsewhere in the 

 Teleostei. 



Apparently the notochord is persistent, and there are long 

 and slender neural and haemal arches, but no ribs. The dermal 

 denticles are uniform in size, and so small as to give a granular 

 appearance to the skin. In structure they are thick, with a 

 flat, enamelled, often sculptured, external surface, quadrate or 

 rhombic in shape, and fitting closely together. Teeth are either 

 absent or very minute, but sometimes (e.g. Acanthodo2^sis and 

 Ischnacanthus) they are few in number and large, conical in 

 shape, occasionally with minute cusps between the larger teeth. 

 Claspers are absent. The Acanthodei are small Fishes, most of them 

 being less than '3 m. in length, and ranging from the Upper Silu- 

 rian to the Lower Permian inclusive. Two families are recognised. 



Fam. 1. Diplacanthidae.^ — Two dorsal fins are present. 

 Usually there is a row of lateral spines extending along each 

 side of the body between the pectoral and pelvic fins. Exclusively 

 Upper Silurian and Devonian. 



The genera Bipl acanthus, Climatius, Parexus, Euthacanthus, 

 and Ischnacanthus are all found in the Lower Old Eed Sandstone 

 of Scotland. Climatius and I)%flacanthv.s are also represented 

 in the Devonian of Canada. 



Fig. 251.^Restoration of Acanthodes wardi. Carboniferous of England and Scotland. 

 (From Smith Woodward.) 



Fam. 2. Acanthodidae. — A single dorsal fin; lateral spines 

 vestigial or absent. Lower Devonian to the Lower Permian. 



