XVII . ELASMOBRANCHII-—ACAN THODEI : 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. Acanthodopsis 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. i 
The genera Diplacanthus, Climatius, Parexus, Euthacanthus, 
and Ischnacanthus are all found in the Lower Old Red Sandstone 
of Scotland. Climatius and Diplacanthus are also represented 
in the Devonian of Canada. 
Fic. 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. 
