136 



OEDEKS OF FISHES. 



NematoptycMus, Gonatodtis, CyclaptycMus, kc,} In the Secon- 

 dary rocks Zepidosteids are extremely abundant, the chief 

 forms belonging to the families Dapedidce, Lcpidotidcc, and 

 Lcptolepidce. In the Dcqyedidce (fig. 502, 1), the tail -fin 

 is only slightly heterocercal, the scales are interlocked by 

 pegs and sockets, and the back teeth are obtuse. Dap)cdius 



Fig. 502. — 1, Dapedius tetra^onolepis ; 2, Leptolepis sprattiformis ; 3, Lepidotus Valdensis. '■ 



itself is compressed and deep - bodied, and is exclusively 

 confined to the Lias. The front teeth in this genus are 

 t}'pically notched or bifurcate. The Jurassic ^chmodus 

 (fig. 503) has been separated from Dapedius upon the 

 ground that the teeth have unicuspidate crowns ; but this 

 alleged distinction has been shown to be neither constant 

 nor reliable, and the former name must therefore be 

 abandoned. The Jurassic genus Tetragonolepis is closely 

 allied to Dapedius, especially in its greatly compressed body 

 and its single long dorsal fin ; but, as shown by Quenstedt 



1 Contrary to the vieAvs which have usually heen held by ichthyologists, 

 Dr Traquair has expressed the opinion that the Palcemiiscidoe (as also the 

 Platysomidm) are more nearly allied to the Acipenseroids than to the Lc])i- 

 dosteidce. 



