214 



OSTEICHTHYES 



girdle (Figs. 241, 263). This consists of a series of paired bones : the 

 clavicle below overlies the coracoid ; the cleithrum strengthens the 

 region where the pectoral fin articulates, covering the scapula ; the 

 uppermost is the post-temporal (suprascapular), which in all typical 

 Osteichthycs unites the dermal shoulder-girdle firmly with the 

 otic region of the skull (Fig. 302). A supratemporal (extrascapular) 

 may occur between the pterotic and the post-temporal. 



All these scales, bones, and lepidotrichia are in primitive forms 



of similar structure, and are pre- 

 sumably of homologous nature. 

 Lying in the connective tissue 

 of the dermis outside the 

 muscles, they are covered 

 over by a layer of mesoblastic 

 tissue and the unbroken epi- 

 dermis. Only the larger bones 

 extend into the deeper layers 

 of the connective-tissue system. 

 True denticles (dermal teeth), 

 similar in structure and develop- 

 ment to those of the Elasmo- 

 branch, are found in some living 

 Teleostomes scattered over the 

 surface of the scales and lepido- 

 trichia. They are either fixed 

 (Lepidosteus, Polypterus, Figs. 

 184, 264) or movably attached 

 to the underlying bone (Silti- 

 roids, Fig. 373). Some fossil 

 forms (Coelacanthidae,Fig. 262) 

 are known to have possessed 

 an abundant supply of similar 

 denticles, and possibly they were 

 widely distributed among early 



Diagram of a section through the dorsal fin of TeleOStomi. 

 a Teleost. a, actinotrich ; I, lepidotrich ; in, 

 radial muscle ; r, endoskeletal fin-radial ; ., 



scale. (From Quart. Joimi. Micr. Set.) What has been the history 



of the ' ganoid ' scale is still an 



undecided question. Since such bony scales occur even in Silurian 

 rocks, it is doubtful whether the clue to their origin will be found 

 in any known fossil. The most favoured theory is that of 

 Williamson [49 60], which has been extended by the researches of 

 O. Hertwig [212] and others (Rohon [369, 371], Pander [312], 

 Gegenbaur [163], etc.). According to Hertwig's view, the history 

 of the scales and plates has been as follows. Starting from a condi- 

 tion, such as still persists in modern Elasmobranchs, where the body 



Fio. 187. 



