The Vertebrate Skeleton

by Sidney H. Reynolds

The order Ganoidei may be divided into three suborders:

(1) Chondrostei. Living genera Acipenser, Scaphirhynchus, Polyodon and Psephurus.

(2) Crossopterygii. Living genera Polypterus and Calamoichthys.

(3) Holostei. Living genera Lepidosteus and Amia.

Suborder (1). Chondrostei.

The skull is immovably fixed to the vertebral column. By far the greater part of the skeleton is cartilaginous. The notochord is persistent and unconstricted, its sheath is membranous, but cartilaginous neural and haemal arches are developed. Intercalary pieces (interdorsalia) occur between the neural arches, and similar pieces (interventralia) between the haemal arches. The cranium is covered with membrane bone, and teeth are but slightly developed. The tail is heterocercal. Gill rays occur on the hyoid arch, and the gills are protected by a bony operculum attached to the hyomandibular. The skin (1) may be almost or quite naked, (2) may carry bony plates arranged in rows, or may be covered (3) with rhomboidal scales, or (4) partly with rhomboidal, partly with cycloidal scales.

Suborder (2). Crossopterygii.

The exoskeleton has the form of cycloidal or rhomboidal scales. The condition of the vertebral column differs in the different genera. Sometimes, as in Polypterus, there are well-developed ossified vertebrae; sometimes, as in many extinct forms, the notochord persists and is unconstricted. The tail may be diphycercal or heterocercal. The pectoral and sometimes the pelvic fins consist of an endoskeletal axis bearing a fringe of dermal rays.

Suborder (3). Holostei.

The exoskeleton has the form of cycloidal or rhomboidal scales. The notochord is constricted and its sheath is segmented and ossified, forming distinct vertebrae, which are generally biconcave, sometimes opisthocoelous (Lepidosteus). The cartilaginous cranium is largely replaced by bone, and in connection with it we find not only membrane bone, but cartilage bone, as the basi-occipital, exoccipitals, and pro-otic are ossified. The tail is heterocercal. The suspensorium resembles that of Teleosteans, consisting of a proximal ossification, the hyomandibular, which is movably articulated to the skull and a distal ossification, the symplectic. The two are separated by some unossified cartilage. The cartilaginous upper and lower jaws are to a great extent surrounded and replaced by a series of membrane bones.

Order IV. Teleostei.

The exoskeleton is sometimes absent but generally consists of overlapping cycloid or ctenoid scales. Bony plates are sometimes present, as in the Siluridae, or the body may be encased in a complete armour of calcified plates, as in Ostracion. Enamel is however never present, and the plates are entirely mesodermal. The skeleton is bony, but in the skull much cartilage generally remains. The vertebral centra are usually deeply biconcave, and the tail is of the masked heterocercal type distinguished as homocercal. In the skull the occipital region is always completely ossified, while the sphenoidal region is generally less ossified. The skull has usually a very large number of membrane bones developed in connection with it. The teeth vary much in character in the different members of the order, but are as a rule numerous and pointed, and are ankylosed to the bone. The suspensorium is hyostylic and the jaws have much the same arrangement as in the Holostei. There are five pairs of branchial arches, of which all except the last bear gill rays. A series of dermal opercular bones is developed in connection with these arches. The pectoral girdle consists almost entirely of dermal clavicular bones. The pelvic girdle has disappeared, its place being taken by the enlarged and ossified dermal fin-rays of the pelvic fins.