ORDERS OF FISHES. 



135 



sented at the present day by the Trout-like " Dog-fishes " 

 {Amia) of tlie Nortli American lakes ; and forms only 

 specifically separable from the recent ones occur in the 

 Tertiary formations of the same country. No pre-Tertiary 

 examples of the group are as yet known ; though it is by 

 no means impossible that some Mesozoic forms may ulti- 

 mately prove to be Amioids. 



Sub-order B. Lepidosteid^. — Scales rhomboidal, not 

 overlapping ; tail heterocercal, sometimes homocercal ; paired 

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

 branchiostegal rays not modified to form "jugular plates." 

 This sub-order is represented at the present day by the 

 Gar-pike {Lcpidosteus, fig. 499, a) of the North American 

 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. The oldest known type is the 

 Cheirolcfpis of the Devonian, which has been shown by Dr 

 Traquair to be allied to Pcdceoniscus. In the Carboniferous 

 and Permian rocks the sub-order is mainly represented by 

 the genera Palcmniscus and Amhlypterus (fig. 501), in which 



Fig. 501. — Rliahdolepis {Amhlypterus) macropterus. Lower Permian. (After Agassiz.) 



the tail is heterocercal, and the jaws are furnished with 

 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. Belonging to the 

 same family as the preceding [Palceoniscidce) are' various 

 genera from the Upper Paliieozoic rocks, such as Pygopterus, 



