LEPIDOGANOIDEI 141 



closely approximated premandibular or dentary elements of 

 the under jaw. 



The Pycnodonts were for the most part deep-bodied fishes, 

 symmetrically compressed from side to side. They were 

 notochordal ; a few of the earlier forms were heterocercal, but 

 the majority of the family were homocercal. 



The pycnodont type was first manifested in the carboni- 

 ferous strata by the heterocercal genus Platysomus, and by the 

 species P. parvulvs, which has been found in that formation 

 at Leeds : but this earliest pycnodont genus is chiefly repre- 

 sented by Permian species, of which Platysomus gibbosus (fig. 

 54) is a fine example. 



In the lias, most beautiful fossil fishes of this group are 

 found, which were referred by Bronn to the genus Tetragono- 

 lejpis, and by Agassiz to the lepidoid sub-order. Sir P. Egerton 

 has, however, shown that the dentition is truly " pycnodont," 

 having a very close resemblance to that of Microdon, but with 

 the masticatory apparatus smaller in proportion to the size of 

 the fish. The scales, moreover, instead of being articulated 

 by interlocking pegs and sockets, as in fig. 52, are joined in a 

 peculiar way, which Sir P. Egerton describes as follows : — 

 " Each scale bears upon its inner anterior margin a thick solid 

 bony rib, extending upwards beyond the margin of the scale, 

 and sliced off obliquely above and below, on opposite sides, 

 for forming splices with the corresponding processes of the 

 adjoining scales. These splices are so closely adjusted, that 

 without a magnifying power, or an accidental dislocation, they 

 are not perceptible. When in situ, and seen internally, these 

 continuous lines decussate with the true vertebral apophyses, 

 and cause the regular lozenge-shaped pattern so characteristic 

 of the pycnodont family."* These decussating " pleurolepidal" 

 lines are, however, confined to the space between the skull 

 and the dorsal fin, as in fig. 55. 



* Proceedings of the Geological Society, May 1853, p. 276. 



