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. [Illustration FIG. 5. SKULL OF A MALE _Chimaera monstrosa_ (after HUBRECHT). 1. nasal capsule. 2. cartilaginous appendage to the fronto-nasal region. 3. erectile appendage. 4. foramen by which the ophthalmic nerves leave the orbit. 5. foramen by which the ophthalmic branch of the Vth nerve enters the orbit. 6. auditory capsule. 7. interorbital septum. 8. mandible articulating with an outgrowth from the posterior part of the palato-pterygo-quadrate. 9. teeth. 10. labial cartilage. II. III. V. VII. IX. X. foramina 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.