The Irregular Fishes are cylindrical, fusi-clavi-spheri-and tubuliform, usually destitute of scales or covered with plates, scutes, and spines. Thus, since the Cartilaginous Fishes are collectively irregular in form, the Irregular Osseous Fishes must be allowed to follow them.
The greatest variety of the osseous system is shown in the limbs, especially the posterior pair, which in the other classes also are generally imperfect and make their appearance the last. In Fishes they are not divided into digits, but only into rays, which probably correspond only to the digital ungues or to feathers. An animal which has fin-rays is assuredly a Fish, for fin-rays do not occur in any Reptile.
The posterior fins change even their situation. Those Fishes in whom they are placed near to the anus, are obviously the more perfect, as the Abdominales.
In others they advance to the rear of the thoracic fins, and are even attached to what has been called the "girdle" or humerus—Thoracici.
In others they even get in front of the thoracic fins on the throat—Jugulares.
Lastly, they are actually wanting—Apodales.
In the Lampreys there are neither thoracic nor ventral fins.
The skeleton of Fishes is not simply divided to a greater extent than in other animals, but has actually a greater number of bones, such as the rays in the perpendicular fins, which are wanting in all animals, even in the Reptilia. An animal with dorsal rays must surely be placed among the Fishes, and consequently the Lepidosiren also.
The misshapen Fishes will therefore occupy the lower stages, the regular the upper, not directly by reason of their form alone; but because the other organs are also more imperfectly developed, the bones being cartilaginous, the tegument asquamous, mucous, or covered with spines, scutes, and plates; the fins wanting or abortive, or displaced from their proper situation; the head disproportionate in size to the body, the mouth wide or narrow, the eyes placed superiorly or upon the forehead.
A lower character is afforded also by very long dorsal fins, such namely as extend from the head to the tail. In the Abdominales, Sturgeons, Sharks, and Rays, the perpendicular fins are small; in the Thoracici, Jugulares, and Apodes they are, on the contrary, mostly very long.