232 



DIPNOI 



small spine-like processes, highly characteristic (Fig. 200, B), and not 

 to be confused with denticles. This spiny covering is found in all 

 the living genera, and even in the Devonian Phaneropleuridae 

 (Fig. 201) ; it is therefore an important and old-established form of 

 not found in any other sub-class. The trabecular and 



isopedine layers remain, 

 though in living genera 

 bone - corpuscles are no 

 longer present (Wieders- 

 heim [490], Klaatsch 

 [264], Goodrich [178]). 



The dermal fin -rays 

 are always jointed, gener- 

 ally branched, and are 

 formed of a bony sub- 

 stance containing bone- 

 cells (Fig. 203). They 

 are slender, much more 

 numerous than the endo- 

 skeletal radials, with an 

 unjointed proximal piece 

 deeply embedded in the 

 connective tissues and 

 muscles, so as to con- 

 siderably overlap the 

 radials [175]. In the 

 modern degenerate Dipnoi 

 the dermal rays, or camp- 

 fitfrichia, become fibrous 

 and little calcified, thus 



FIG. 202. 



somewhat resembling the 



Diagram of a section through the dorsal fin of ceratotrichia 

 Dipterus. b.sc, body-scales ; (Hst.r, distal jointed region , i -p, , . 



of the camptotrich ; efr, radial of fin ; pr.r, proximal Ol'ancnS. iSUt in the early 

 unsegmented region of the camptotrich ; s.sc, scale over- 

 lying dermal ray. (From Quart. Joitrn. Micr. Sci.) 



of Elasmo- 

 branchs. 



fossils they are thoroughly 

 bony, and approach lepi- 

 dotrichia in structure. The distal- jointed region of the camptotrichia 

 is, however, always covered over externally by true scales ; and in 

 Dipterus these fit closely in rows along the rays (Figs. 202, 226). 

 It is, therefore, possible either that in Dipnoi we have lepidotrichia, 

 which have sunk deep below the surface, and been covered over by 

 a new set of scales ; or that the camptotrichia really are modified 

 ceratotrichia, with which the more superficial true scales are be- 

 coming associated. A still closer union of these two elements might 

 have given rise to the typical lepidotrichia of primitive Teleostomes. 

 Whatever may have been the history of the camptotrichia, they 

 differ considerably from the dermal rays of other fish [175]. 



