A treatise on zoology . r-girdle firmly with theotic region of the skull (Fig. 302). A supratemporal (extrascapular)may occur between the pterotic and the post-temporal. All these scales, bones, and lepidotrichia are in primitive forms of similar structure, and are pre-sumably of homologous in the connective tissueof the dermis outside themuscles, they are coveredover by a layer of mesoblastictissue and the unbroken epi-dermis. Only the larger bonesextend into the deeper layersof the connective-tissue denticles (dermal teeth),similar in structure and develoj)-ment to t
A treatise on zoology . r-girdle firmly with theotic region of the skull (Fig. 302). A supratemporal (extrascapular)may occur between the pterotic and the post-temporal. All these scales, bones, and lepidotrichia are in primitive forms of similar structure, and are pre-sumably of homologous in the connective tissueof the dermis outside themuscles, they are coveredover by a layer of mesoblastictissue and the unbroken epi-dermis. Only the larger bonesextend into the deeper layersof the connective-tissue denticles (dermal teeth),similar in structure and develoj)-ment to those of the Elasmo-branch, are found in some livingTeleostomes scattered over thesurface of the scales and lepido-trichia. They are either fixed{Lepidosteus, Polypterus, , 264) or movably attachedto the underlying bone (Silu-roids. Fig. 373). Some fossilforms (Goelacanthidae,Fig. 262)are known to have possessedan abundant supply of similardenticles, and possibly they werewidely distributed among Fig. 187. Diagram of a section through the dorsal fin ofa Teleost. «, actinotrich ; I, lepidotrich ; m,radial muscle; /, endoskeletal fin-radial ; .--?,scale. (From Qtiart. Jonrn. Mia: Sci.) What has been the historyof the ganoid scale is still anundecided question. Since such bony scales occur even in Silurianrocks, it is doubtful whether the clue to their origin will be foundin any known fossil. The most favoured theory is that ofWilliamson [496(;], which has been extended by the researches of0. Hertwig [212] and others (Kohon [369, 371], Pander [312],Gegenbaur [163], etc.). According to Hertwigs view, the historyof the scales and plates has been as follows. Starting from a condi-tion, such as still persists in modern Elasmobranchs, where the body SCALES 215 is covered with isolated placoid scales, it is supposed that thedenticles developed extensive basal plates in the dermis; by thecoalescence of adjoining basal plates were formed larger plates orscales st
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