Page:The American Cyclopædia (1879) Volume VII.djvu/626

 614 GANOIDS ' double ; gills free and lying in an operculated cavity, with or without an opercular gill, a pseu- do-branchia, and blowing holes ; the arterial trunk always with numerous valves ; no decus- sation of the optic nerves, and the ventral fins abdominal ; there is always an air bladder, and a duct communicating with the oasophagus ; the eggs are conveyed from the abdominal cavity by tubes ; like the shark family, they have a thymus gland, and often a spiral valvu- lar fold in the intestine. Miiller divides the ganoids into four families : 1, containing the American gar fish; 2, the polypterus of Afri- ca ; 3, the amia, or mud fish of America ; and 4, the sturgeons ; these will be described under the first, third, and fourth titles respectively. Prof. Agassiz is inclined to elevate the ganoids from an order to a class, separate from ordinary bony fishes, and superior to them in organiza- tion, though inferior to the selachians (sharks and rays) ; he makes them the third class of the branch vertebrata, with the three orders of coelacanths, acipenseroids (sturgeons), and sau- roids (gar fish), with three additional doubtful orders of siluroids, plectognaths (balistes, ostra- cion, and porcupine fishes), and lophobranchs (hippocampus, pipe fish, and pegasus). The re- mainder of this article will be devoted to the consideration of fossil ganoids, and to the in- teresting questions connected with their struc- ture and geological distribution. Ganoid fossil scales, whether angular, rhomboidal, or many- sided, are imbricated like the slates of a roof, and formed of an outer plate of enamel, an in- ner porcelain layer, and an intermediate reticu- lated structure analogous to the diploe between the tables of the human skull. The fin rays of the ganoids are bare enamelled bones, each con- sisting of a plate of enamel on each side and a lamina of bone between them ; the necessary flexibility of such rays is secured by joints which extend through the rigid enamel, leaving the central bony plate undivided, on the principle of the half-sawed moulding which the carpen- ter wishes to bend at an angle or around some curved surface. The dermal or external skele- ton of the ganoids is so remarkably developed, that in many instances it has served to deter- mine the forms of genera and species in the old red standstone and carboniferous strata, every other portion having perished, as their internal skeletons were either very slight or entirely cartilaginous ; enamelled plates repre- sent the head, enamelled imbricated scales in- dicate the form and proportions of the body, and enamelled rays show the position and out- line of the fins. The instances of great devel- opment of the outer skeleton in higher animals are few, being limited principally to the arma- dillo and pangolin among mammals, the tor- toises and crocodiles among reptiles, and the gar pikes and sturgeons among fishes. The earliest fishes, those of the Silurian epoch, seem to have been all placoids ; in the following age, that of the old red sandstone, the ganoids ap- peared in great numbers, and with the placoids formed the entire class of fishes for unknown millions of years, that is, through the old red sandstone, carboniferous, Permian, triassic, and oolitic periods, down to the cretaceous epoch, when ordinary bony fishes were brought into existence ; the age of these two orders corre- sponding nearly to the reign of the ferns, palms, coniferous trees, and their allies among plants. When the bony fishes became the prevailing type, the other orders diminished in proportion, so that at the present time the gar pikes and the sturgeons are the chief representatives of the old powerful and numerous ganoids. In the human family we see similar instances of nations reaching their culminating point, and then disappearing or lapsing into barbarism ; the Aztecs of Central America and the Copts of Egypt are the remnants of the great races which built the mounds of the Mississippi val- ley and the Egyptian temples and pyramids. In the words of Hugh Miller: " But in the rivers of these very countries, in the polypterus of the Nile or the lepidosteus of the Mississippi, we are presented with the few surviving fragments of a dynasty compared with which that of Egypt or Central America occupied but an ex- ceedingly small portion of either space or time. The dynasty of the ganoids was at one time coextensive with every river, lake, and sea, and endured during the unreckoned eons which extended from the times of the lower old red* sandstone until those of the chalk." Among ganoids are found some of the strangest ichthyic forms, having a structure and placoid affinities no longer seen in nature ; these gigantic and strange fishes were of the first rank in their class, and, being then the only representatives of the vertebrates, exhibited characters belong- ing to the higher class of reptiles, of which they were the prophetic types. These reptilian fishes attained their greatest number and lar- gest size during the carboniferous period, and were remarkable both for their formidable offensive weapons and their strong defensive armor. This remote age was as fully charac- terized by bloodshed and destruction of animal life as any since the creation of man ; indeed, no animal ever had more powerful teeth than the ganoid rJiizodus of the coal fields, sharper than and four times as large as the hugest liv- ing crocodile possesses,; the dorsal and caudal spines of some of the contemporary placoids far exceeded in size and destructive powers those of any living shark or ray ; where such weapons were employed, defensive armor was necessary ; hence the enamelled scales of the ganoids. In the lepidoid or acanth family, confined chiefly to the old red sandstone and carboniferous strata, the teeth are small, brush- like, and in several rows, or obtuse and in a single row; the scales are flat, rhomboidal, parallel to and wholly covering the body ; all those occurring in strata earlier than the Juras- sic have the tail heterocercal or with the spine prolonged into its upper lobe ; it has no repre- sentative among living fishes. Among the most