Page:An Introduction to the Study of Fishes.djvu/77

Rh The vertebræ are either abdominal or caudal vertebræ, the coalescence of the parapophyses into a complete hæmal ring, and the suspension of the anal fin generally forming a sufficiently well-marked boundary between abdominal and caudal regions (Fig. 23). In the Perch there are twenty-one abdominal and as many caudal vertebræ. The centrum of the first vertebra or atlas is very short, with the apophyses scarcely indicated, and lacking ribs like the succeeding vertebra. All the other abdominal vertebræ, with the exception of the last or two last, are provided with ribs, many of which are bifid (72). A series of flat spines (74), called interneurals, to which the spines and rays of the dorsal fins are articulated, are supported by the neural spines, the strength of the neurals and interneurals corresponding to that of the dermal spines (75). The caudal vertebræ, differ from the abdominal in having the hæmapophyseal elements converted into spines similar to the neurals, the anterior being likewise destined to support a series of interhæmals (79), to which the anal rays are articulated. The last and smallest caudal vertebra articulates with the hypural (70), a fan-like bone, which, together with the dilated hindmost neural and hæmal elements, supports the caudal rays.

Looking at a perch's skull from the side (Fig. 24), the most superficial bones will be found to be those of the jaws, a chain of thin bones round the lower half of the eye, and the opercles.

The anterior margin of the upper jaw is formed by the intermaxillary or premaxillary (17) which bears teeth, terminates in a pedicle above, to allow of a forward sliding motion of the jaw, and is dilated into a flat triangular process behind, on which leans the second bone of the upper jaw, the maxillary (18). This bone is toothless, articulates with the vomer and palatine bone, and is greatly dilated towards its distal