[a]Fig. 5. Skull of a male] Chimaera monstrosa (after [a]Hubrecht]).
| 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. |
These singular fish have the skin smooth and in living forms almost or quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are fused to the cranium, and Meckel's cartilage articulates directly with the part corresponding to the quadrate. The skull is distinctly articulated with the spinal column, the notochord is persistent and unconstricted, and the skeletogenous layer shows no trace of metameric segmentation, though in the neural arches this segmentation is readily traceable. The neural arches of the first few vertebrae are fused together and completely surround the notochord, while they do not in other parts of the body. The tail is diphycercal. Of the living genera, in Callorhynchus there is no trace of calcification in the skeletogenous layer, while in Chimaera rings of calcification are found, there being three to five for each vertebra as indicated by the foramina for the exit of the spinal nerves. The pelvic fins are produced into claspers. Besides the living genera Chimaera, Harriotta and Callorhynchus a fair number of fossil forms are known, e.g. Ischyodus.
Order III. Ganoidei.
The fishes included under the term Ganoidei form a very heterogeneous group, some of which closely approach the Dipnoi, others the Elasmobranchii, others the Teleostei. The great majority of them are extinct, only eight living genera being known; these are all inhabitants of the northern hemisphere, and with the exception of Acipenser, which is both fluviatile and marine, are entirely confined to fresh water.
The following is a list of the living genera of Ganoids with their respective habitats:—
Acipenser. Rivers and seas of the northern hemisphere.
Scaphirhynchus. Mississippi and rivers of Central Asia.
Polyodon (Spatularia). Mississippi.
Psephurus. Yan-tse-kiang, and Hoangho.
Polypterus. Rivers of tropical Africa.
Calamoichthys. Some rivers of West Africa.
Lepidosteus. Freshwaters of Central and North America and Cuba.
Amia. Rivers of Carolina.
The exoskeleton is very variable, thus the body may be:—
(a) Naked or with minute stellate ossifications as in the Polyodontidae. (b) Partially covered with large detached bony plates as in Scaphirhynchus and Acipenser. (c) Entirely covered with rhomboidal ganoid scales as in Lepidosteus, Polypterus, Palaeoniscus and many extinct forms. (d) Covered with rounded scales shaped like the cycloid scales of Teleosteans as in Amia. (e) Having the trunk and part of the tail covered with rhomboidal scales, and the remainder of the tail with rounded scales as in Trissolepis.
The teeth also are very variable. The endoskeleton shows every stage of transition from an almost entirely cartilaginous state as in Acipenser to a purely bony state as in Lepidosteus. Sometimes, as in Acipenser, the notochord persists, and its sheath is unsegmented; sometimes, as in Lepidosteus, there are fully formed vertebrae. The tail may be heterocercal, as in Acipenser, or diphycercal as in Polypterus. The cartilaginous cranium is always covered with external membrane bone to a greater or less extent, and the suspensorium is markedly hyostylic. The pectoral girdle is formed of two parts, one endoskeletal and cartilaginous, corresponding with the pectoral girdle of Elasmobranchs, and one exoskeletal and formed of membrane bones, corresponding with the clavicular bones of Teleosteans. The pelvic fins are always abdominal. The fins often, as in Polypterus, have spines (fulcra) attached to their anterior borders.