scale enclosing canal 



Figure 8-40. Lateral-line scales of a perch as seen in surface view. A, and in longitudinal section, B. 



enamel and dentine do not overlay the old; rather, the old 

 surface is first removed (resorbed) then a new surface 

 applied. This type of replacement is a continual process and 

 scales frequently are in a transitional condition with a part 

 of their surface removed. 



The primitive cosmoid scale was rhomboidal in shape 

 but without peg and socket. There was some overlapping 



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accumulation of scleroblasts (only nuclei shown) 



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isopedine or fibrous layer 

 Figure 8-41. Three semidiogrammatic stages in the development of 

 a scale in the teleost. 



with adjacent scales. In the several choanate lines— Rhipi- 

 distia, Actinistia, and Dipnoi, the rhomboid shape has been 

 replaced by the cycloid. Concurrently, there was a reduc- 

 tion in thickness and an histological simplification. In 

 Eusthenopteron, the surface tubercles are bony; below these is 

 a layer of spongy bone and below this a layer of laminar 

 bone. A cosmoid scale of cycloid form, still retaining some 

 of the enamel-covered dentine tubercles, is observed in the 

 coelacanth Lalimena. 



Among the dipnoans, the scales originally were of the 

 cosmoid type with enamel, dentine, and bony layers. In 

 Dipierus, the earliest known of the dipnoans (Middle Devo- 

 nian), the scales are already cycloid in form but are other- 

 wise comparable to the osteolepid. With time, the outer 

 enamel and dentine layers have been lost, and, in forms such 

 as Neoceratodus, only spongy and laminar bone layers remam. 

 Protopterus has the scales covered by many small denticles, 

 whose bases flare out as small plates which attach to a 

 laminar but largely acellular bony plate much like that of 

 the teleost. 



The exact agreement in scale and dermal bone construc- 

 tion between Dipierus and the Crossopterygii suggests a 

 common origin. The cosmoid scale contrasts sharply with 

 the ganoid scale of the actinopterygian or other fishes. 



Amphibians 



The scale structure of early amphibians is not well known. 

 It appears from the dermal bones of the skeleton that they 

 were entirely bony. There was a tuberculate outer lami- 

 nated part, a middle spongy zone, and laminated base. 



SCALES 



239 



