FAMILY VI. SAURICHTHYIDE.
Magnificent species of heterocercal rhomb-scaled Ganoids, with large dispersed laniary teeth, sometimes of a size rivalling those of great Saurians, for which they have been mistaken, have left their remains in the coal strata at Carluke, near Glasgow, and other localities, and constitute the genus Mega lichthys of Agassiz. The head is defended by strong ganoid plates, of a beautiful polish ; the trunk-scales are usually granulate exteriorly. In this genus, as in the type of the family, the fulcra of the fin-rays are in two rows : all the known species of Sauriclithys are triassic.
Pygopterus and Acrolepis, with fin-fulcra in a single row ; Eurynotus, Elonichthys, Plectrolepis, Graptolepis, Orognathus, Pododus, Acanthodes, and Di plapterus, are carboniferous genera of Ganoids, with rhomboid scales. Ceelacanthus, Isodus, Phyl lolepis, Hoplopygu,s, Uronemus, Colemodus, Centrodus, Asterolepis, Psammosteus, and 0 ste-oplax, are genera of Ganoids with rounded scales, represented by species in carboniferous strata.
Of the above-named genera, Acroltpis, PygTterus, Paleeo niscus, and Ccelacanthus, continue to be represented in Permian beds ; in which also are found species of the ganoid genera Dorypterus, Holaeanthodus, and Globulodus, if the teeth on which the latter is based be not those of Platysomus, a pycno dont genus which is both Permian and carboniferous.
The formations of the mezozoic or secondary periods give evidence of the full development of the ganoid order. In the lowest or " triassic" division this order is still represented by heterocercal and notochordal species belonging to some of the genera of the Permian period, as, e. g., Ccelacanthlui, Arablyp term, and Palteoniseus. The genus Placodus, a supposed pycnodont fish of the muschelkalk, has been shown to be a conchivorous Saurian.* In the oolitic division the heterocercal Ganoids are almost completely superseded by homocercal genera, which now, for the first time, appear on the stage of life ; but the ossification of the endoskeleton is still incomplete. In the cretaceous series the Teleostian, or well-ossified, bony fishes, are nume rous ; and here also first are seen fishes with the flexible " cycloid" or " ctenoid" scales, and of genera which continue to be represented by living species.
Of 33 genera of fishes in the lias, 4 only were represented in older strata, while the rest extend into the upper oolitic beds. Of these, 19 genera are Ganoids with rhomboid scales, and two (Leptolepis and Gyrosteus) have rounded scales.