. The Cambridge natural history. Zoology. 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 slen
. The Cambridge natural history. Zoology. 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 ( 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 Biplacanthus, Climatius, Parexus, Euthacanthus, and Ischnacanthus are all found in the Lower Old Eed Sandstone of Scotland. Climatius and I)% are also represented in the Devonian of 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 Please note that these images are extracted from sca
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
