. Chordate morphology. Morphology (Animals); Chordata. denticle. ^i>^ Figure 8-39. Structure of teleost scales. A, section of scale of teuciscus tu^\\\j%; B, joint between two armor plates of a catfish Hypostoma; C, section of scale and skin of Arapaima gigas; D, surface layers of a scale of the catfish, Ca/lichfhyes tongifillis. (A after Hose; B after Rauther; C and D after O. Hertwig) scale (this is an ontogenic progression from the rhombid interrelationship to the cycloid). The anterior margin is ex- tended forward below the next anterior scale. Growth is largely marginal, and continues
. Chordate morphology. Morphology (Animals); Chordata. denticle. ^i>^ Figure 8-39. Structure of teleost scales. A, section of scale of teuciscus tu^\\\j%; B, joint between two armor plates of a catfish Hypostoma; C, section of scale and skin of Arapaima gigas; D, surface layers of a scale of the catfish, Ca/lichfhyes tongifillis. (A after Hose; B after Rauther; C and D after O. Hertwig) scale (this is an ontogenic progression from the rhombid interrelationship to the cycloid). The anterior margin is ex- tended forward below the next anterior scale. Growth is largely marginal, and continues throughout life. The surface of the scale is decorated with small circular ridges marking the growth periods. In Lepisosleus, the lateral-line scales ossify first in the tail, then in an anterior progression (Figure 8-42). From this line, scale rows extend up and forward and down and back (or down and forward). The caudal peduncle is fully cov- ered by scales before they appear above, or below, the lateral line anteriorly. The development of the scales paral- lels that of Polypterus. In most teleosts, the first scale papillae or buds appear along the lateral line on the caudal peduncle and later ex- tend forward along that line. In centrarchids, the row of scale primordia extends forward below the lateral line; this appears to be a deviation from the primitive style. Choanate fishes The crossopterygian fishes have a scale identified by the term cosmoid (Williamson, 1849) referring to the "decora- tive" nature of the bunches of dentinal tubules in these scales. In this type, the surface is covered with enamel-clad tubercles joined one with another to form a surface broken only by pores (Figure 8-43). Below the enamel is a layer of dentine, the cosmine, in which the dentinal tubules radiate outward and upward from the central pulp cavity of each tubercle. Below the dentine is spongy bone permeated by many anastomosing channels, and below this is lamellar bone. On the an
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