A treatise on zoology . hes, and similar haemal arches thickened at thebase of the hypochordal fin. Traces have also been found(Dean [HI]) of dorsal radials forming a row separate from thevertebral column. Eadials also appear in the front region of thehypochordal fin strengthened with dermal scales. The skull andvisceral arches seem to have closely resembled those of theumphistylic Elasmobranchs (Fig. 159). The palato-quadrate hasa large otic process, the hyomandibular is elongated, the fivebranchial arches have the usual epi-, cerato-, hypo-, and basi-branchial elements. Most remarkable is th
A treatise on zoology . hes, and similar haemal arches thickened at thebase of the hypochordal fin. Traces have also been found(Dean [HI]) of dorsal radials forming a row separate from thevertebral column. Eadials also appear in the front region of thehypochordal fin strengthened with dermal scales. The skull andvisceral arches seem to have closely resembled those of theumphistylic Elasmobranchs (Fig. 159). The palato-quadrate hasa large otic process, the hyomandibular is elongated, the fivebranchial arches have the usual epi-, cerato-, hypo-, and basi-branchial elements. Most remarkable is the strengthening of theprimary upper and lower jaws with hard bone-like tissue. Thistissue is probably never typical bone ; but it resembles it very closely,. 190 ACANTHODII and contains numerous cells which may have branching marginal teeth of the jaws, sometimes large and multicuspid,are firmly fixed to these bones in some genera (Fig. 160).As a rule, the teeth are small, or absent. Sometimes, however, cp. Fig. 150. Diagram of the skull and visceral arches of an Acauthodian. hr, fifth branchial arch ; hxn,hyomandibular ; , calcified plate bearing similar branchial rays (?); ih, Meckels cartilage;, otic process of the palato-quadrate ; po, ethmoid firocess. they are of considerable size, and Dean has described in hclma-eanthus rows of teeth, with large central and small lateral cusps,resembling those of primitive sharks (Cladoselachians, etc.). Theceratohyal seems to be covered with a plate bearing a series (ortwo series, Rohon [3G7]) of long branchial rays, all of similar hard
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