Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/664

646 646 ICHTHYOLOGY [OSTEOLOGY, are sometimes observed on the haemal arches. Ribs are either absent or but imperfectly represented (Carcharias). The substance of the skull of the Chondropterygians is cartilage, interrupted especially on its upper surface by more or less extensive fibre-membranous fontanelles. Superficially it is covered by a more or less thick shagreen- Fig. 32. Fig. 33. FIGS. 32, 33, 34. Lateral view, longitudinal section, and transverse section of caudal vertebra of Basking Shark (Seiache maxima). (After Hasse.) a, cen trum ; b, neurapophysis ; c, intercrural cartilage ; d, hsemapophysis ; e, spinal canal ; /, intervertebral cavity ; g, central canal for persistent portion of notochord ; !i, hseinal canals for blood-vessels. like osseous deposit. The articulation with the vertebral column is effected by a pair of lateral condyles. In the sharks, besides, a central conical excavation corresponds to that of the centrum of the foremost vertebral segment, whilst in the rays this central excavation of the skull receives a condyle of the axis of the spinous column. The cranium itself is a continuous undivided cartilage, in which the limits of the orbit are well marked by an anterior and posterior protuberance. The ethmoidal region sends horizontal plates over the nasal sacs, the apertures of which retain their embryonic situation upon the under surface of the skull. In the majority of Chondropterygians these plates are produced conicaily, forming the base of the soft projecting snout; and in some forms, especially in the long-snouted rays and the saw-fishes (Pristis), this prolongation appears in the form of three or more tubi- form rods. As separate cartilages there are appended to the skull a suspensorium, a palatine, a mandible, a hyoid, and rudimentary maxillary elements. The suspensorium is movably attached to the side of the skull. It generally consists of one piece only, but in some rays of two. In the rays it is articulated to the mandible only, their hyoid possessing a distinct point of attachment to the skull. In the sharks the hyoid is suspended from the lower end of the suspensorinm together with the mandible. What is generally called the upper jaw of a shark is, as Cuvier has already stated, not the maxillary, but the palatine. It consists of two simple lateral halves, each of which arti culates with the corresponding half of the lower jaw, which is formed by the simple representative of Meek el s cartilage. Some cartilages of various sizes are generally developed on each side of the palatine, and one on each side of the mandible. They are called labial cartilages, and seem to represent maxillary elements. The hyoid consists generally of a pair of long and strong lateral pieces, and a single mesial piece. From the former, cartilaginous filaments (representing branchiostegals) pass directly outwards. Branchial arches, varying in number, and similar to the hyoid, succeed it. They are suspended from the side of the foremost part of the spinous column, and, like the hyoid, bear a number of filaments. The vertical fins are supported by interneural and inter- hasmal cartilages, each of which consists of two or more pieces; to these fins the fin-rays are attached without articulation. The scapular arch of the sharks is formed by a single coracoid cartilage bent from the dorsal region downwards and forwards. In some genera (Scyllium, Squatina) a small separate scapular cartilage is attached to the dorsal extremities of the coracoid ; but in none of the Elasmobranchs is the scapular arch suspended from the skull or vertebral column ; it is merely sunk and fixed in the substance of the muscles. Behind, at the point of its greatest curvature, three carpal cartilages are joined to the coracoid, which Gegenbaur has distinguished as propterygium, mesopterygium, and metapterygium, the first occupying the front, the last the hind margin of the fin. Several more or less regular transverse series of styliform cartilages follow. They represent the phalanges, to which the horny filaments which are imbedded in the skin of the fin are attached. In the rays, with the excep tion of torpedo, the scapular arch is intimately connected with the confluent anterior portion of the vertebral column. The anterior and posterior carpal cartilages are followed by a series of similar pieces, which extend like an arch for wards to the rostral portion of the skull, and backwards to the pubic region. Extremely numerous phalangeal elements, longest in the middle, are supported by the carpals, and form the skeleton of the lateral expansion of the so-called disk of the ray s body, which thus, in. fact, is nothing but the enormously enlarged pectoral fin. The pubic is represented by a single median transverse cartilage, with which a tarsal cartilage articulates. The latter supports the fin-rays. To the end of this cartilage is also attached, in the male Chondropterygians, a peculiar accessory generative organ or clasper. The Holocephali differ from the other Chondropterygians in several important points of the structure of their skeleton, and unmistakably approach certain Ganoids. That their spinal column is persistently notochordal has been men tioned already. Their palatal apparatus, with the suspen sorium, coalesces with the skull, the mandible articulating with a short apophysis of the cranial cartilage (autostylic skull). The mandible is simple, without anterior symphysis. The spine with which the dorsal fin is armed articulates with a neural apophysis, and is not immovably attached to it, as in the sharks. The pubic consists of two lateral halves, with a short, rounded tarsal cartilage. The skeleton of the Ganoid fishes presents extreme varia tions with regard to the degree in which ossifications replace the primordial cartilage. Whilst some exhibit scarcely any advance beyond the Plagiostomes with persistent cartilage, others, as regards the development and specialization of the several parts of their osseous framework, approach the Teleosteans so closely that their Ganoid nature can be demonstrated by, or inferred from, other considerations only. All Ganoids possess a separate gill-cover. The diversity in the development of the Ganoid skeleton is well exemplified by the few representatives of the order in the existing fish fauna. Lowest in the scale in this respect are those with a persistent notochord, and an autostylic skull, that is, a skull without separate suspen sorium the fishes constituting the suborder Dipnoi, of which the existing representatives are Lepidosiren, Proto- pterus, and Ceratudus, and the extinct (so far as demonstrated at present) Dipterus, Chirodus (and Phaneropleuron 1). In these fishes the notochord is persistent, passing uninter ruptedly into the cartilaginous base of the skull. Some Dipnoi are diphy-, others hetero-cercal. Neural and heemal elements and ribs are well developed. The primordial cranium of the Dipnoi is cartilaginous, but with more or less extensive ossifications in its occipital, basal, or lateral portions, and with large tegumentary bones, which, from this suborder upwards in the series, will be found to exist throughout the remaining forms of fishes. A strong process descends from the cranial cartilage, and offers by means of a double condyle (fig. 35, s) attachment to corresponding articulary surfaces of the lower jaw. Maxil-