In like fashion the spines and rays of the anal fin (18) are jointed at base to bones called _interhæmals_ (77). In certain cases the second interhæmal is much enlarged, made hollow and quill-shaped, and in its concave upper end the tip of the air-bladder is received. This structure is seen in the plume-fishes (_Calamus_). These two groups of bones, interneural and interhæmal, are sometimes collectively called _inter-spinals_. The flattened basal bone of the _caudal fin_ (80) is known as _hypural_ (79). [Illustration: FIG. 34.--Basal bone of dorsal fin, _Holoptychius leptopterus_ (Agassiz). (After Woodward.)] The tail of the striped bass, ending in a broad plate which supports the caudal, is said to be homocercal. In more primitive forms the tail is turned upward more or less, the fin being largely thrown to its lower side. Such a tail as in the sturgeon is said to be heterocercal. In the isocercal tail of the codfish and its relatives the vertebræ are progressively smaller behind and the hypural plate is obsolete or nearly so, the vertebræ remaining in the line of the axis of the body and dividing the caudal fin equally. The simplest form of tail, called diphycercal, is extended horizontally, tapering backward, the fin equally divided above and below, without hypural plate. In any form of the tail, it may through degeneration be attenuate or whip-like, a form called leptocercal. =The Pectoral Limb.=--The four limbs of the fish are represented by the paired fins. The anterior limb is represented by the pectoral fin and its basal elements with the shoulder-girdle, which in the bony fishes reaches a higher degree of complexity than in any other vertebrates. It is in connection with the shoulder-girdle that the greatest confusion in names has occurred. This is due to an attempt to homologize its parts with the shoulder-girdle (scapula, coracoid, and clavicle) of higher vertebrates. But it is not evident that a bony fish possesses a real scapula, coracoid, or even clavicle. The parts of its shoulder-girdle are derived by one line of descent from the undifferentiated elements of the cartilaginous shoulder-girdle of ancestral crossopterygian or dipnoan forms. From a similar ancestry by another line of differentiation has come the amphibian and reptilian shoulder-girdle and its derivative, the girdle of birds and mammals. =The Shoulder-girdle.=--In the higher fishes the uppermost bone of the shoulder-girdle is called the _post-temporal_ (_suprascapula_) (53). In the striped bass and in most fishes this bone is jointed to the temporal region of the cranium. Sometimes, as in the trigger-fishes, it is grown fast to the skull, but it usually rests lightly with the three points of its upper end. In sharks and skates the shoulder-girdle, which is formed of a continuous cartilage, does not touch the skull. In the eels and their allies, it has, by degradation, lost its connection and the post-temporal rests in the flesh behind the cranium.