dentici 



denticles 



mucous 

 canal 



enamel 



dentine 



(cosmine) 

 pulp cavity 



spongy bone 



B 



laminated bone (isopedine) 



lar bone 



_-~-_ _~ ^ \_^ spongy 



ngy bone 



^ V V 





; 1"-; spongy bone 



lamellar bone 



vascular canal 



Figure 8-43. Scale structure in choanate fishes, the cosmoid scale. A, stereodiagram of the ante- 

 rior margin and section of a scale of Poro/ep/s ura/ensis,- B, surface appearance and articulation of 

 scales of Poro/ep/s urolens/s,- C, section of dermal bone of Eus/henopteron,- D, section of scale of 

 Neocerotodus. (A, B, C after Bystrow, 1 939 and 1 959) 



branched. They are composed of a fibrous horny material 

 with bone cells in their proximal part, and with an outer 

 calcified layer which extends some distance distally (Figure 

 8-46). The ceratotrichia of Lepidosiren are soft, have few 

 joints, and are unbranched, more like those of the shark. 



In the Rhipidistia, the lepidotrichia are like those of Dip- 

 terus: bony, jointed, branched, and hollowed on their inner 

 aspect, but they are also covered with enamel and dentine 

 in the more primitive types (Porolepis, Gyroptychivs) where 

 there is a gradual transition from the body scales to those 

 of the lepidotrichia. 



This transition is misleading since it involves two distinct 

 entities: the lepidotrichium which continues into the fleshy 

 base of the fin to overlap the radial and the outer body cover 

 of the scales. The transition suggests that the outer cover of 

 scales has been fused with or continued by the scales of the 

 lepidotrichium. In the advanced forms, this break becomes 

 well marked {Eusthenopleron). The nature of this break sug- 

 gests that in the thinning of the fleshy fin, the lepidotrichia 

 have become exposed by the loss of an outer layer of scales 

 and have taken the place of that layer. This event is better 

 explained ontogenetically, in terms of a thin fin extending 



FIN RAYS • 241 



