The _caudal_ fin is rarely symmetrical, so that its upper half would be equal to its lower; the greatest degree of asymmetry obtains in fishes with heterocercal termination of the vertebral column (see subsequently, Figs. 31, 41). In fishes in which it is nearly symmetrical it is frequently prolonged into an upper and lower _lobe_, its hind margin being concave or more or less deeply excised; in others the hind margin is rounded, and when the middle rays greatly exceed in length the outer ones the fin assumes a pointed form. [Illustration: Fig. 4.--Labrax lupus (Bass), an Acanthopterygian with anterior spinous, and posterior soft dorsal fin.] Many and systematically important differences are observed in the _dorsal_ fin, which is either spiny-rayed (spinous) (_Acanthopterygian_), or soft-rayed (_Malacopterygian_). In the former, a smaller or greater number of the rays are simple and without transverse joints; they may be flexible, or so much osseous matter is deposited in them that they appear hard and truly spinous (Fig. 3); these spines form always the anterior portion of the fin, which is detached from, or continuous with, the remaining jointed rays. The spines can be erected or depressed at the will of the fish; if in the depressed position the spines cover one another completely, their points lying in the same line, the fish is called _homacanth_; but if the spines are asymmetrical, alternately broader on one side than on the other, the fish is called _heteracanth_. The spinous division, as well as the one consisting of jointed rays, may again be subdivided. In the _Malacopterygian_ type all the rays remain jointed; indeed, sometimes the foremost ray, with its preceding short supports, is likewise ossified, and a hard spine, but the articulations can nearly always be distinctly traced. Sometimes the dorsal fin of Malacopterygian fishes is very long, extending from the head to the end of the tail, sometimes it is reduced to a few rays only, and in a few cases it is entirely absent. In addition to the rayed dorsal fin, many Malacopterygian fishes (as the Salmonoids, many Siluroids, Scopeloids, etc.) have another of greater or lesser extent, without any rays; and as always fat is deposited within this fold, it is called a _fatty_ fin (_pinna adiposa_). [Illustration: Fig. 5.--Saurus undosquamis, a Malacopterygian with anterior soft dorsal, and additional adipose fin.] The _anal_ fin is built on the same plan as the dorsal, and may be single or plural, long or short, or entirely absent; in Acanthopterygians its foremost rays are frequently simple and spinous. The _horizontal_ or _paired_ fins consist of two pairs: the pectorals and ventrals.