THE ELASMOBRANCHII AND HOLOCEPHALI. 385 



naked-skinned, modern representatives. The primitive elasmobranchs quickly 

 acquired a fish-like form, losing the extensive ancestral peribranchial chamber, 

 and at no phase of their development showing, so far as known, any trace of 

 a tadpole stage. 



The notochord, at an early period, is invested and largely replaced by well- 

 developed cartilaginous centra, and an elaborate system of more or less calcined 

 cartilaginous gill bars, and neural and haemal arches is developed. 



The lateral fold is generally retained for a longer or shorter period, giving 

 rise by local enlargement to well defined pectoral and pelvic fins. The parietal 

 eye is not conspicuously developed; neither are the three pairs of embryonic 

 oral arches, nor the corresponding appendages. True teeth appear on the upper 

 and lower margins of the mouth, but they are not preceded by any recognizable 

 dental plates. An air bladder is absent. 



The ova are very large, fertilized within the body, and the males are provided 

 with highly specialized intromittent organs or claspers. 



The elasmobranchs, owing largely to their well developed sensory and loco- 

 motor organs, and to their formidable jaws and teeth, developed rapidly in efficiency 

 during the late palaeozoic period, but they pass the climax of their evolution 

 without producing a noticeably higher type of organization. While the internal 

 skeleton may be highly developed and more or less calcified, it never develops 

 into true bone. 



The elasmobranchs are pelagic or deep water fishes rather than frequenters 

 of the shallow brackish waters of the shore. 



The absence of an air-bladder excluded the possibility of their becoming 

 air breathers, and the pectoral and pelvic fins show no signs of developing into 

 elongated, digitate appendages suitable for locomotion on land. 



The Holocephali probably arose from the ccelolepid branch of the ostraco- 

 derms, developing along somewhat similar lines as the elasmobranchs, but 

 retaining certain features of the parent stock not seen in the latter. The noto- 

 chord is persistent, but enveloped by ring-like calcifications of its sheath more 

 numerous than the cartilagenous neural and haemal arches. They retain the 

 large head and small trunk, or tadpole form, of the ostracoderms; a peribranchial 

 chamber, and a short body cavity, with the viscera and anus placed well forward. 

 Three pairs of dental plates are present, consisting of vascular dentine and grow- 

 ing from persistent pulps. They probably represent the premaxillary, maxillary, 

 and mandibular plates of the ostracoderms and arthrodires, and belong to the 

 three pairs of primitive oral arches seen in amphibian embryos. The general 

 shape of the mouth and the form of the jaws resemble those of the ostracoderms 

 and amphibian tadpoles rather than those of an elasmobranch. No true verte- 

 brate teeth are developed. 



The primitive characters above mentioned are those of the ostracoderms 

 and are sufficient to distinguish the chimaeras from all other adult vertebrates. 

 But the cartilaginous internal skeleton, the large-sized ova, the internal fertiliza- 

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