104 



PALAEONTOLOGY. 



closely approximated premanclibular 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. parvulus, which has been found in that formation 



at Leeds : but this 

 earliest pycnodont 

 genus is chiefly re- 

 presented by Per- 

 mian species, of 

 which Platysomus 

 gibbosus (fig. 73) is a 

 fine example. 



In the lias, many 

 beautiful fossil fishes 

 of this group 



are 



Fig. 73. 

 Platysomus gibbosus (zechstein of Mansfield) 



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

 lepis. Sir P. Egerton has shewn that the dentition is " pycno- 

 dont," having a close resemblance to that of Microdus, 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. 70, 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 



