One species (C. antarcticus) is common in the Southern temperate zone. Cunningham describes the egg (see Fig. [81], p. 169), as being of a dark greenish-black colour, and, in general, measuring from eight to nine or even ten inches in length, by about three in breadth. It consists of a central, somewhat spindle-shaped convex area (between the horny walls of which the young fish lies), surrounded by a broad plicated margin, which is fringed at the edge, and covered on the under surface with fine light brownish-yellow hairs.
SECOND ORDER—GANOIDEI.
Skeleton cartilaginous or ossified. Body with medial and paired fins, the hinder pair abdominal. Gills free, rarely partially attached to the walls of the gill-cavity. One external gill-opening only on each side; a gill-cover. Air-bladder with a pneumatic duct. Ova small, impregnated after exclusion. Embryo sometimes with external gills.
To this order belong the majority of the fossil fish remains of palæozoic and mesozoic age, whilst it is very scantily represented in the recent fauna, and evidently verging towards total extinction. The knowledge of the fossil forms, based on mere fragments of the hard parts of the body only, is very incomplete, and therefore their classification is in a most unsatisfactory state. In the following pages only the most important groups will be mentioned.
[For a study of details we have to refer to Agassiz, “Poissons Fossiles;” Owen, “Palæontology,” Edinb. 1861, 8vo; Huxley, “Preliminary Essay upon the Systematic Arrangement of the Fishes of the Devonian Epoch,” in Mem. Geolog. Survey, Dec. 10; Lond. 1861, and “Illustrations of the Structure of Crossopterygian Ganoids,” ibid. December 12, 1866; Traquair, “The Ganoids of the British Carboniferous Formations,” part I. “Palæoniscidæ.” Palæontogr. Soc. Lond. 1877.]
Eight suborders may be distinguished at present.
FIRST SUB-ORDER—PLACODERMI.
Extinct. The head and pectoral region of the body encased in great bony, sculptured plates, with dots of enamel; the remainder of the body naked, or with ganoid scales; skeleton notochordal.
Comprises the oldest vertebrate remains, from Devonian and Carboniferous formations. Pterichthys: (Figs. [135] and [136]), tail tapering, covered with small ganoid scales, without caudal fin; the cephalic shield was probably moveably joined to the cuirass of the trunk, and both were composed of several pieces; the abdominal shield consisted of one single median plate, and two pairs of lateral plates, a third small pair being sometimes observed detached in front of the anterior pair; pectoral exceedingly long, consisting of two pieces movably connected with each other; tail scaly, and short; a small dorsal fin placed on the tail; a pair of small ventrals; jaws small, with confluent denticles. Several species have been distinguished in remains found in the strata of Caithness and other localities in Scotland. Coccosteus (Fig. [137], p. 354): all the bony plates are firmly united, no pectoral spines; tail naked and long; a dorsal and anal fin supported by interneural and interhæmal spines. Dentition unknown. Dinichthys: a gigantic fish from the Devonian of North America (estimated at from 15 to 18 feet in length), with the dermal covering very similar to that of Coccosteus, but with a simple arched dorsal shield. As in this latter genus the caudal extremity does not possess external or internal bony parts, and the ventral plastron of both genera corresponds in every particular; the dentition is so singularly like that of Lepidosiren, that Newberry (Geolog. Survey of Ohio, vol. ii. part 2) considers this genus to be in genetic relation to the Dipnoi. The following genera have been united in a separate family, Cephalaspidæ; viz. Cephalaspis: head covered by a continuous shield with tubercular surface, produced into a horn at each posterior corner; a median dorsal backward prolongation bears a spine; heterocercal. Auchenaspis and Didymaspis: allied to the preceding, but with the cephalic shield divided into a larger anterior and smaller posterior piece. Pteraspis: with the cephalic shield finely striated or grooved, composed of seven pieces. Scaphaspis and Cyathaspis: with the surface of the head-shield similarly sculptured as in Pteraspis, but simple in the former, and composed of four pieces in the latter. Astrolepis: attained to the gigantic size of between twenty and thirty feet; its mouth was furnished with two rows of teeth, of which the outer ones were small, the inner much larger.
