GANOIDEI. 323 



The position of at least two of these sub-orders (viz., Acan- 

 thodidce and Ostracostei) in the order of the Ganoids is question- 

 able ; whilst the Sturionidce have been referred elsewhere. In 

 any case, the number of forms included in these sub-orders is 

 so large that nothing more can be done here than simply to 

 draw attention to some of the more striking examples of each. 



SUB-ORDER A. AMIABLE. In this sub-order are included 

 Ganoids in which the scales are rounded and overlap one an- 

 other, and the tail is homocercal. The vertebral column is 

 ossified, and the external appearance approaches closely to 

 that of an ordinary Teleostean fish. A few fossil forms from 

 the Tertiary Rocks have been more or less doubtfully referred 

 to this group ; and the sub-order is represented at the present 

 day by various American fishes, all belonging to the genus 

 Amia. 



SUB -ORDER B. LEPIDOSTEID^E. Scales rhomboidal, not 

 overlapping ; tail heterocercal, sometimes homocercal ; paired 

 fins not lobate ; fin-borders generally with fulcral scales ; true 

 branchiostegal rays. This sub-order is represented at the 

 present day by the Gar-pike (Lepidosteus) of the North Ameri- 

 can continent, and it attained its greatest development in the 

 Mesozoic period. The exact range of the sub-order in time is 

 uncertain, as it has not yet been determined what forms should 

 be included in it. If Cheirolepis be excluded, the sub-order is 

 not represented at all in the Devonian Rocks. In the Car- 

 boniferous and Permian Rocks the sub-order is mainly repre- 

 sented by the genera Palaoniscus and Amblypterus (fig. 283), in 

 which the tail is heterocercal, and the jaws are furnished with 



Fig. y&^Amblypter 



numerous minute teeth. Numerous species of these genera are 

 known in the above-mentioned formations, and both appear for 

 the last time in the Trias. In the Secondary Rocks Lepido- 

 steids are extremely abundant, the chief forms belonging to the 



