The Hyostylic. In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch. These terms are more fully discussed in Chapter VIII.
There is always an internal framework supporting the gills; it usually consists of the hyoid arch and five, rarely six or seven, pairs of branchial arches. The limbs are represented by two pairs of fins, the pectoral and the pelvic; they are not divided into proximal, middle and distal portions. The ribs do not unite with a median ventral sternum, or meet in the mid-ventral line in any other way in the trunk region.
Order I. Elasmobranchii.
The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen. In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel. These spines may be imbedded in the flesh in front of the paired or unpaired fins, or may be attached to the tail. They are specially characteristic of the suborder Acanthodii. The endoskeleton is cartilaginous and true bone is never found. Much of the skeleton, especially of the vertebral column, is however often calcified, this being especially well seen in the anterior part of the vertebral column of Rays (Raiidae). In living forms cartilaginous biconcave vertebrae are always well developed, but in some extinct forms the notochord persists unconstricted. Neural and haemal arches are however always developed; they sometimes remain separate, sometimes fuse with the centra. Ribs are often wanting and when present are often not separated off from the vertebrae. The cranium is a simple cartilaginous box whose most prominent parts are the capsules which enclose the sense organs. The skull is sometimes immovably fixed to the vertebral column, sometimes articulates with it by means of two condyles. There is no operculum and no representative of the maxilla or premaxillae. The teeth are very variable. Large pectoral and pelvic fins always occur.
The Elasmobranchii may be divided into four suborders:—
(1) Ichthyotomi.
(2) Pleuropterygii.
(3) Selachii.
(4) Acanthodii.
Suborder (1). Ichthyotomi[30].
The members of this suborder range from the Devonian to the Permian and so have long been extinct.
The endoskeletal cartilage has granular calcifications evenly distributed throughout it. The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender. There is a continuous dorsal fin with separate basalia and radiale. The tail is diphycercal, and the pectoral fins are typical archipterygia[31]. The pelvic fins of the male are prolonged to form claspers.
The best known of these primitive Elasmobranchs are the Pleuracanthidae.
Suborder (2). Pleuropterygii.
This suborder was formed for the reception of Cladoselache, an Elasmobranch found in the Lower Carboniferous of Ohio[32].
The exoskeleton is in the form of small, thickly-studded dermal denticles. The vertebral centra are unossified, and the tail is strongly heterocercal. There were certainly five, perhaps seven gill slits, and the suspensorium is apparently hyostylic. The paired fins are, according to the view which derives them by concentration from continuous lateral folds, the most primitive known (see p. 129) and claspers are absent.
Suborder (3). Selachii.
Cartilaginous or partially calcified biconcave vertebrae are always well developed; they constrict the notochord intervertebrally. The neural and haemal arches and spines are stout and intercalary cartilages (interdorsalia) are present. The tail is heterocercal, but in some cases (Squatina) approaches the diphycercal condition. In most cases the suspensorium is hyostylic, the jaws being attached to the cranium by means of the hyomandibular, and the palato-pterygo-quadrate bar not being fused to the cranium. There are generally five pairs of branchial arches, and gill rays are borne on the posterior surface of the hyoid arch, and on both the anterior and posterior surfaces of the first four branchial arches. The Notidanidae differ from most Selachians in two respects, first as regards the suspensorium—Meckel's cartilage articulating directly with the palato-pterygo-quadrate bar, and not being connected with the hyoid arch; and secondly as regards the number of branchial arches—six pairs occurring in Hexanchus and seven in Heptanchus.
The pectoral fins are without the segmented axis of the archipterygium. In most cases they are sharply marked off from the body and lie almost at right angles to it; but in the Rays they have the form of lateral expansions in the same plane as the body, from which they are not sharply marked off. The pelvic fins in the male bear long grooved cartilaginous rods which are accessory copulatory organs or claspers.
There are two principal groups of Selachii, the Squalidae or Sharks and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have the shape of ordinary fish, the pectoral fins are vertically placed and the body ends in a powerful heterocercal tail. The Batoidei have flattened bodies owing to the great size and horizontal position of the pectoral fins. The tail is long and thin and is often armed with spines. The teeth in Selachii differ much in character in the different forms, and are always arranged in numerous rows. They are generally pointed and triangular or conical in the Squalidae, while in the Batoidei they are often broad and flattened.
Suborder (4). Acanthodii.
The fishes included in this group are all extinct and in some respects are intermediate between Elasmobranchii and Ganoidei. The body is elongated and closely covered with small scales consisting of dentine enamelled at the surface. The notochord is persistent and the calcification of the endoskeletal cartilage is only superficial. The tail is heterocercal. The jaws bear small conical teeth, or in some cases are toothless. The skeleton of all the fins differs from that of modern Elasmobranchs in having the cartilaginous radiale much reduced, and the fins are nearly always each provided with an anterior spine, which except in the case of the pectoral fins is merely inserted between the muscles. These spines are really enormous dermal fin-rays; the pectoral fin-spine is articulated to the pectoral girdle.
The suborder includes many well-known extinct forms like Acanthodes and Diplacanthus; it ranges from the Devonian to the Permian.
PISCES, HOLOCEPHALI.
Order II. Holocephali.
This order includes a single suborder only.
Suborder. Chimaeroidei.
| 1. nasal capsule. | 6. auditory capsule. |
| 2. cartilaginous appendage to | 7. interorbital septum. |
| the fronto-nasal region. | 8. mandible articulating with |
| 3. erectile appendage. | an outgrowth from the posterior |
| 4. foramen by which the | part of the palato-pterygo-quadrate. |
| ophthalmic nerves leave the orbit. | 9. teeth. |
| 5. foramen by which the | 10. labial cartilage. |
| ophthalmic branch of the Vth nerve | II. III. V. VII. IX. X. foramina |
| enters the orbit. | for the passage of cranial nerves. |