PYCNODONTEB.
The name of this group of ganoid fishes refers to the blunt rounded form of the greater proportion of the teeth, especially those attached to the palate and hind alveolar part of the lnwar inar • tha anterior teeth are small and sub-prehen sile ; but the whole dentition bespeaks fishes adapted to feed on small testaceous and crustaceous mals. In the modern " Sea Breams " (Spa Platysomus gibbogua (zechstein of Mansfield). roids), with an analo gous dentition, the two premaxillaries oppose the two mandibulars, but in the extinct Pycnodonts the vomer, as in Anarhichas, opposes its pavement of teeth to that of the two • By this term is meant a symmetrical shape of the tail fin, with an unsym metrical development of the supporting spines, the terminal vertebrfe inclining to the upper lobe.
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. parinclus, which has been found in that formation at Leeds : but this earliest pycnodont genus is chiefly repre sented by Permian species, of which Platysomus (fig. 54) is a fine example.
In the liar, most beautiful fossil fishes of this group are found, which were referred by Bronn to the genus Tetragono lepis, 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 These decussating " pleurolepidal" lines are, however, confined to the space between the skull and the dorsal fin, as in fig. 55.
The Pycnodonts so characterized are further distinguished from the closely-resembling lepidoid genus Dapedius, by having the small anterior teeth conical and single-pointed, instead of being bifurcate ; and although this character is subject to occasional variations, nevertheless, on taking a comprehensive view of all the dapedioid species, it seems to have been suffi ciently constant to warrant the continuance of the separation of the group into the unicuspid and bicuspid species. And Sir P. Egerton has accordingly proposed to apply the generic terms .Echraodus (from aix/hi, a point, and 'odour, a tooth),* for the unicuspid and pycnodont species, formerly termed Tetra gonolepis, and to continue the name Dapedius for the bicuspid and unequivocally lepidoid homocercal deep-bodied Ganoids, many beautiful species of which are found in the lias.