XX] SCALES 483 



another in the mid-ventral line. Each half of the girdle is ossified 

 by two bones, a scapula above (sometimes called the hyper- 

 coracoid) and a coracoid (sometimes termed the hypocoracoid) 

 below. In fact, the real fulcrum for the play of the fin is not 

 afforded by the vestigial girdle but by a series of overlying mem- 

 brane bones to which the girdle is attached. This chain begins 

 above with a forked post-temporal bone attached to crests on the 

 epiotic and supra-occipital bones of the skull, then follows a supra- 

 cleithrum, then acleithrum, a curved bony bar to the under-side 

 of which the scapula is attached, and projecting backwards from the 

 lower end of this a postcleithrum. This last is not always 

 present (Fig. 237). The pelvic girdle is represented by a single 

 bone on each side. 



The well-known scales with which the bodies of Teleostei are 

 covered are in the overwhelming majority of cases thin films of 

 dentine which are embedded in sacs of the dermis. Originally 

 however these scales were of essentially the same nature as the 

 membrane bones which cover the head and the pectoral girdle, and 

 in some of the most primitive families a layer of bone is found 

 forming the deeper part of the scale. Such scales with or without 

 the bony layer are termed cycloid if they have a rounded posterior 

 border, ctenoid if the posterior border is serrated. In fossil 

 Teleostei the scales not only had the bony layer below but they 

 were also covered above by a layer of hard shining material called 

 ganoin, which is structureless material exhibiting neither the 

 lacunae of bone nor the dentinal canals of dentine. Such scales 

 are called ganoid scales. In one family of living fish (S. American 

 Siluroids) the scales appear to be covered with ganoin, and in these 

 fish the surface of the scale is beset with small placoid denticles and 

 the scales are large (probably as a result of the fusion of smaller 

 scales), and are denominated scutes. Similar bony scutes but 

 without the placoid denticles or the ganoin are found in some other 

 living families of Teleostei. In many Teleostei (in the Herring 

 family for instance) as the fish grows the scales grow by the 

 addition of concentric rings round their edges. From an inspection 

 of the scales it is possible to calculate the age of these fish, and 

 this becomes of great importance when the question of fish-migra- 

 tion is studied. It appears probable that scales and membrane 

 bones first arose as ossifications of the dermis the purpose of which 

 was to connect together placoid scales or teeth, and it must be 

 remembered that teeth are merely enlarged placoid scales lining 



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