They are cycloid or ctenoid according as their projecting margin is entire or denticulated. Among Plectognathi (Teleostei) the Ostraciontidae are covered with a mail of large polygonal plates; and the genus Diodon with its immediate allies has projecting spines with bare skin between. Bony shields of great size are found in many living Siluroidei, but especially in the extinct Ganoid Placodermi (Pterichthys, Cephalaspis) from Devonian and Carboniferous strata, but they are restricted to the fore-part of the body
1 Pristis, the Saw-fish, one of the Rays, has a long flat prae-nasal cartilage. A linear series of teeth, similar in structure to the oral teeth of Myliobatis, another Ray, are implanted in sockets along the edge of this rostrum. They grow from persistent pulps. The Saw-fish is said to glide rapidly by a whale and to rip it with these weapons.
The dermal exoskeleton appears to have given origin to the investing bones of the skull and shoulder-girdle, which adapt themselves more and more to the underlying cartilage, and eventually replace it in part in the former. The fin-rays supporting the azygos fins and the edges of the paired fins have a similar origin. Each ray primitively consists of a series of pieces, which are paired right and left in the azygos fins, dorsal and ventral in the paired.
The continuous azygos fin of the embryo persists in the adult of some Fish, e. g. the Teleostean Eel, Sole, Cod (Anguilla, Pleuronectes, Gadus) probably by reversion (Balfour). It is usually broken up into one or more dorsals, a caudal, and one or more anals. The fin-rays are horny in Elasmobranchii; cartilaginous throughout in chondrostean Ganoidei and the Dipnoi, but with a thin shell of bone formed in the perichondrium in the latter; whilst in Teleostei and bony Ganoidei they are ossified. In the latter case they may be soft and consist either of a series of joints or be branched dichotomously; or entire, i.e. spinose, and of one piece throughout. In some Elasmobranchii and in Polypterus the sections of the dorsal fin are carried each by a strong spine (ichthyodorulite); in a few Teleostei its posterior lobe contains only horny fin-rays, is soft and fatty, forming a small adipose fin (Salmonidae, some Siluroidei, &c). The fin-rays of the ventral portion of the caudal fin are carried by haemal arches; but those of its dorsal portion, of the dorsal and anal fins by skeletal elements lying in the median fibrous septum dividing the right and left halves of the body (see p. 101). The caudal fin, which is an all-important organ of locomotion, is either heterocercal, homocercal, or diphycercal.
It is heterocercal when the vertebral axis is bent dorsally and terminates in a lobe or point, and the caudal ventral lobe is placed at a greater or less distance from its extremity; homocercal when it is outwardly symmetrical, but in reality consists almost entirely of a caudal ventral lobe, the extension of the vertebral axis beyond the lobe being atrophied; and diphycercal when it is divided into equal dorsal and ventral lobes by a straight vertebral axis. A heterocercal caudal fin is found in Elasmobranchii, Holocephali, chondrostean and many extinct Ganoidei; a homocercal in living bony Ganoidei, and the majority of Teleostei; and a diphycercal in a few Teleostei and the Dipnoi (see p. 97). In some Ganoidei, living, e. g. Acipenser, Lepidosteus, and extinct, a single or double row of small spines known as fulcra extend along the anterior margins of the dorsal or caudal fin, and on the ventral edge of the latter as well in Lepidosteus.
The chondrocranium is always well-developed. It is massive in the Sturgeon and some Teleostei, e.g. Salmon, and its prae-nasal region is occasionally of great size, e. g. in some Rays, as Pristis, the Sturgeons, and a few Teleostei. It is interrupted by superior fontanelles in Amia; by lateral and basal fontanelles as well in Polypterus and Lepidosteus (Ganoidei). There are neither investing bones nor intrinsic centres of ossification in Elasmobranchii and Holocephali. In the latter the palato-pterygo-quadrate cartilage is continuous with the cranium; among the former it articulates with it in Hexanchus and Heptanchus (Notidanidae), whilst in others it is attached by ligament and by the hyomandibular1. A spiracular cartilage is sometimes found in the edge of the ligament anterior to the spiracle (infra). The lower jaw is a simple rod of cartilage articulating with the palato-pterygo-quadrate and connected to the cranium by the hyomandibular. In other Fish both intrinsic centres of ossification and investing bones are found, and, taking a Teleostean (p. 91) as a standard, the following points may be noted.
The jaw-apparatus of Acipenser is only connected to the cranium by a hyomandibular and a symplectic cartilage, both partly ossified: the metapterygoid region of Lepidosteus has an articulation with the base of the skull; and the palato-pterygo-quadrate is continuous with the cranium in Dipnoi, forming also the sole support of the mandible. Acipenser possesses only a pro-otic, ali- and orbito-sphenoid and an ecto-ethmoid ossification in the cranium, and the mandibular cartilage is unossified as it is also in Dipnoi. Among the bony Ganoidei there is no supra-occipital, no pterotic: in Polypterus no epiotic distinct from the opisthotic. The prae-pituitary region contains in Amia two ali-, two orbito-sphenoids and ecto-ethmoids: in Lepidosteus two ali-sphenoids and a single bone, perhaps formed by coalesced ecto-ethmoids: in Polypterus two sphenoidal bones and ecto-ethmoids. The palatine and pterygoid bones of Lepidosteus are simply investing bones, as in many Amphibia. Polypterus has no symplectic. The lower jaw has more than one articular ossification, and in Amia a mento-meckelian. The Dipnoi have exoccipitals and palato-pterygoid ossifications. As to the investing bones of the skull there are very many variations.
Acipenser has a large number, among which may be recognised a supra-occipital, paired parietals and frontals, and a huge parasphenoid. There are additional bones, not seen in Teleostei, to be found in bony Ganoidei, such as a squamosal over the pterotic region, a supra-temporal bone in Amia, etc. Protopterus among Dipnoi has a fronto-parietal, a median supra-ethmoid (as in the Salmon), two nasals, and a large supra-orbital on each side; Ceratodus an unpaired parietal and frontal and a pair of lateral bones, the outer perhaps a prae-opercular. The vomer is double in bony Ganoidei, absent in Dipnoi; in both the parasphenoid is large and extends backwards beneath the backbone as it does in Acipenser. The praemaxillae are absent in the Sturgeons: the maxillae are represented by a chain of bones in Lepidosteus, and both praemaxillae and maxillae are absent in Dipnoi. The mandible is invested by a dentary bone alone in Sturgeons: by dentary, splenial, and angular in Dipnoi, and Polypterus, to which Lepidosteus and Amia, add a supra-angular and the former a coronoid.