two principal pairs of teeth are borne on the palato-pterygoids and splenials, while the third pair are found in the vomerine region. The tail is diphycercal in living forms. In the extinct Dipteridae it is heterocercal. The pectoral girdle includes both membrane and cartilage bones. The pelvic girdle consists of a single bilaterally symmetrical piece of cartilage.
This suborder is represented by the living genera Ceratodus, Protopterus and Lepidosiren, and among extinct forms by the Dipteridae and others.
Suborder (2). Arthrodira.
Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown.
The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera are Coccosteus from the European Devonian and Dinichthys, a large predatory form from the lower Carboniferous of Ohio.
CHAPTER VI.
THE SKELETON OF THE DOGFISH[34].
Scyllium canicula.
I. EXOSKELETON.
The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales.
A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin.
II. ENDOSKELETON.
The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue.
The endoskeleton is divisible into an axial portion consisting of the vertebral column, skull, and skeleton of the median fins, and an appendicular portion consisting of the skeleton of the paired fins and their girdles.
1. The Axial Skeleton.
A. The Vertebral Column and Ribs.
The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.
These vertebrae are developed round an unsegmented rod, the notochord, which forms the axial support of the embryo. The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform. The vertebrae are divided into two groups, an anterior group of trunk vertebrae, and a posterior group of caudal or tail vertebrae.
A typical vertebra consists of a middle portion, the centrum, a dorsal portion, the dorsal or neural arch, which surrounds the spinal cord, and a ventral portion, the ventral or haemal arch, which similarly encloses a space.
The tail vertebrae of the dogfish have this typical arrangement, the trunk vertebrae have the haemal arches modified.
Each centrum is a short cylinder of cartilage surrounding an hourglass-shaped cavity occupied by the notochord. The neural arches are composed of three separate elements, the vertebral neural plates (basidorsalia), intervertebral neural plates (interdorsalia), and neural spines (supradorsalia).
The vertebral neural plates are in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. The intervertebral neural plates are polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the spinal nerves.
The neural spines are small patches of cartilage filling up the gaps between the dorsal ends of the neural plates.
The haemal arches (basiventralia) differ much in the trunk and tail portions of the vertebral column. In the trunk portion the centra are flattened below, and the two halves of the haemal arch diverge from one another as blunt ventri-lateral processes to which short cartilaginous rods, the ribs, are attached. Further back at about vertebra 37, the two halves of the haemal arch project downwards and meet forming a complete arch. Further back still, towards the hind end of the tail, the haemal arches bear median haemal spines (ventrispinalia).
B. The Skull.
The skull of the dogfish remains cartilaginous throughout the life of the animal, and has consequently a far more simple structure than have the skulls of higher animals, in which complication has been produced by the development of bone.
The skull consists of the following parts:—
(1) a dorsal portion, the cranium, which lodges the brain, and to the sides of which the capsules of the auditory and olfactory sense organs are united. The cranium may be compared to an unsegmented continuation of the vertebral column;
(2) a number of ventral structures, disconnected or only loosely connected with the cranium. These together constitute the visceral skeleton forming the jaws and supporting the gills.
(1) The Cranium.
The Cranium is an oblong box, with a flattened floor and a more irregular roof. Its sides are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits.
(a) On the dorsal surface of the cranium the following points should be noticed. First at the anterior end, the large thin-walled nasal or olfactory capsules (fig. 6, 1), each of which is drawn out into a narrow cartilaginous process.
The olfactory capsules have no ventral walls, and are separated from one another by the internasal septum, which is drawn out into a third slender process. These three processes together constitute the rostrum (fig. 6, 2).
Behind the olfactory capsules comes a large, nearly circular, hole, the anterior fontanelle, slightly behind which are the two ophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and suborbital ridges. Behind are the auditory capsules (fig. 6, 8), each of which is marked by a pair of prominent ridges, converging towards the middle line to a pair of apertures. These apertures communicate with two canals, the aqueductus vestibuli, which lead into the internal ear. The two ridges lodge respectively the anterior and posterior vertical semicircular canals of the ear.
(b) The principal structures to be noted in a side view of the cranium are contained in the orbit or eye-cavity. Near the base of the orbit at its anterior end is seen the small orbitonasal foramen (fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is the large ophthalmic foramen (fig. 6, 5) so prominent in a dorsal view