A treatise on zoology . ^ osscas, U A, surface view of trunk-scales from which the skin and connectivetissue^ have been removed on the left side. B, longitudinal section through the skin andscales, , anterior articulating process; cj, connective tissue of cutis (7, denticle ),dorsal articulatin- process; ep, epidermis; g, ganoine layer; sk, scale; skn, skin eo\enngscales ; , vascular canal. an extension of it inwards. In the Osteichthyes, on the contrary,denticles are developed separately, often later than the underlyingbony plate, and only secondarily become connected with
A treatise on zoology . ^ osscas, U A, surface view of trunk-scales from which the skin and connectivetissue^ have been removed on the left side. B, longitudinal section through the skin andscales, , anterior articulating process; cj, connective tissue of cutis (7, denticle ),dorsal articulatin- process; ep, epidermis; g, ganoine layer; sk, scale; skn, skin eo\enngscales ; , vascular canal. an extension of it inwards. In the Osteichthyes, on the contrary,denticles are developed separately, often later than the underlyingbony plate, and only secondarily become connected with , the basal plate of a placoid scale is never formed of truebone, but either of dentine or of some allied substance. Thirdly,the scale often sinks away from the epidermis, and grows by theaddition of new layers over its surface (cp. p. 356). The shinyganoine layer of the scales of the Osteichthyes, being formed by themesoblast, must not be confused with true enamel deposited by theepidermis. 2l6 0S7EICH7HYES. SCALES 217 According to Williamsons original theory the scales and i)latesof the Osteichthyes have been formed by the confluent aggrega-tion and superficial depression of a number of placoid teeth over-lying and becoming fused with a lower bony plate. We have seenhow admirably this theory accounts for the structure of theexoskeleton of the Pteraspidomorphi (p. 19-3) ; will it e^pially welliiccount for that of the ganoid scale ? W^e must carefully distinguish between two \ery difterent kinds ofscales hitherto called ganoid (Goodrich [178]). The first, which isthe true (janoid scale, occurs in the Actinopterygii. The second may
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
