A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Fig. 373.—Basal bone of dorsal fin, Holoptychius leptopterus (Agassiz). (After Woodward.)

In the Ordovician rocks of Cañon City, Colorado, Dr. Walcott finds numerous bony scales with folded surfaces and stellate ornamentation, and which he refers with some doubt to a Crossopterygian fish of the family Holoptychiidæ. This fish he names Eriptychius americanus. If this identification proves correct, it will carry back the appearance of Crossopterygian fishes, the earliest of the Teleostome forms, to the beginning of the Silurian, these Cañon City shales being the oldest rocks in which remains of fishes are known to occur. In the same rocks are found plates of Ostracophores and other fragments still more doubtful. It is certain that our records in palæontology fall far short of disclosing the earliest sharks, as well as the earliest remains of Ostracophores, Arthrodires, or even Ganoids.

Megalichthyidæ.—The Megalichthyidæ (wrongly called "Rhizodontidæ") have the pectoral fins obtuse, the teeth relatively simple, and the scales cycloid, enameled. There are numerous species in the Carboniferous rocks, largely known from fragments or from teeth. Megalichthys, Strepsodus, Rhizodopsis, Gyroptychius, Tristichopterus, Eusthenopteron, Cricodus, and Sauripterus are the genera; Rhizodopsis sauroides from the coal-measures of England being the best-known species.

The Osteolepidæ differ from the Megalichthyidæ mainly in the presence of enameled rhomboid scales, as in Polypterus and Lepisosteus. In Glyptopomus these scales are sculptured, in the others smooth. In Osteolepis, Thursius, Diplopterus, and Glyptopomus a pineal foramen is present on the top of the head. This is wanting in Parabatrachus (Megalichthys of authors). In Osteolepis, Thursius, and Parabatrachus the tail is heterocercal, while in Diplopterus and Glyptopomus it is diphycercal. Osteolepis macrolepidotus and numerous other species occur in the Lower Devonian. Diplopterus agassizii is common in the same horizon. Megalichthys hibberti is found in the coal-measures, and Glyptopomus minimus in the Upper Devonian. Palæosteus is another genus recently described.

Fig. 374.—Gyroptychius microlepidotus Agassiz. Devonian. Family Megalichthyidæ. (After Pander.)

The Onychodontidæ are known from a few fragments of Onychodus sigmoides from the Lower Devonian of Ohio and Onychodus anglicus from England.

Fig. 375.—Cœlacanthus elegans Newberry. From the Ohio Carboniferous, showing air-bladder. (After Dean.)