Sub-Grade 3. OSTEICHTH YES. 



The remainder of the true fish are included in this sub-grade. 

 Many important characters distinguish them from the Chondrich- 

 thyes, which remain at a lower grade of organisation. The cartila- 

 ginous endoskeleton becomes to a considerable extent reduced in 

 the adult, and replaced by true bone, or some bone-like tissue 

 derived from true bone (p. 355). 



The exoskeleton is also modified and strengthened by the 

 development of bony tissues. Whilst the primitive placoid scales 

 (denticles) may remain, at all events in certain regions, the body 

 is protected by the development of a new kind of bony scale or 



Fig. 1S2. 



Ltitr'isrtis rutilus. Riyht-siile view, .showing the scales. .Some of the lines of .scales Lave been 

 marked in black, to demonstrate their correspondence with the myotomes. S, lateral line. 

 (After A. Hase.) 



plate below them. The structure of these plates is very variable ; 

 and their exact ontogenetic and phylogenetic relation to the over- 

 lying denticles is often difficult to understand. On the trunk and 

 tail of the lower Osteichthyes are generally found thick, shiny, 

 more or less rhomboid scales in oblique rows corresponding numeri- 

 cally to the myotomes (Figs. 182, 183, 193). Each scale to some 

 extent overlaps its neighbours from before backwards like the tiles 

 of a roof, and the anterior deep -lying edge is often produced into 

 an articulating process. In the higher forms the scales become 

 thinner, more deeply imbricating, less closely articulated, and may 

 lose their metameric disposition. 



Agassiz [4] classified the fish into the groups Placoidei, 

 Ganoidei, Cycloidei, and Ctenoidei, according to the character of 



210 



