1. Vomer. 4. Frontal. 7. Epiotic. 9. Pterotic. 11. Exoccipital. 13. Parasphenoid. 16. Alisphenoid. 3. Prefrontal. 5. Sphenotic. 8. Supraoccipital. 10. Opisthotic. 12. Basioccipital. 15. Prootic.

Names of Bones of Fishes.—In the study of the names of the bones of fishes it will be more convenient to begin with a highly specialized form in which each of the various structures is present and in its normal position.

To this end we present a series of figures of a typical form, choosing, after Starks, the striped bass (Roccus lineatus) of the Atlantic coast of the United States. For this set of plates, drawn from nature by Mrs. Chloe Lesley Starks, we are indebted to the courtesy of Mr. Edwin Chapin Starks. The figures of the striped bass illustrate a noteworthy paper on "The Synonymy of the Fish Skeleton," published by the Washington Academy of Sciences in 1901.

Bones of the Cranium.—The vomer (1) is the anterior part of the roof of the mouth, armed with small teeth in the striped bass and in many other fishes, but often toothless. The ethmoid (2) lies behind the vomer on the upper surface of the skull, and the prefrontal (3) projects on either side and behind the ethmoid, the nostrils usually lying over or near it and near the nasal bone (51). Between the eyes above are the two frontal (4) bones joined by a suture. On the side behind the posterior angle of the frontal is the sphenotic (5) above the posterior part of the eye. Behind each frontal is the parietal (6). Behind the parietal and more or less turned inward over the ear-cavity is the epiotic (7). Between the parietals, and in most fishes rising into a thin crest, is the supraoccipital (8), which bounds the cranium above and behind, its posterior margin being usually a vertical knife-like edge. The pterotic (9) forms a sort of wing or free margin behind the epiotic and over the ear-cavity. The opisthotic (10) is a small, hard, irregular bone behind the pterotic. The exoccipital (11) forms a concave joint or condyle on each side of the basioccipital (12), by which the vertebral column is joined to the skull. The parasphenoid (13) forms a narrow ridge of the roof of the mouth, connecting the vomer with the basioccipital. In some fishes of primitive structure (Salmo, Beryx) there is another bone, called orbitosphenoid, on the middle line above and between the eyes. The basisphenoid (14) is a little bone above the myotome or tube in which runs the rectus muscle of the eye. It descends toward the parasphenoid and is attached to the prootic. The prootic (15) is an irregular bone below the ear region and lying in advance of the opisthotic. The alisphenoid (16) is a small bone in the roof of the mouth before the prootic. These sixteen bones (with a loose bone of specialized form, the otolith, within the ear-cavity) constitute the cranium. All are well developed in the striped bass and in most fishes. In some specialized forms they are much distorted, coossified, or otherwise altered, and their relations to each other may be more or less changed. In the lower forms they are not always fully differentiated, but in nearly all cases their homologies can be readily traced. In the sharks and lampreys the skull constitutes a continuous cartilaginous box without sutures. In the dipnoans and other forms having a bony casque the superficial bones outside the cranium may not correspond to the cartilaginous elements of the soft skull itself.

Fig. 25—Roccus lineatus. Posterior view of cranium.

Bones of the Jaws.—The bones of the jaws are attached to the cranium by membranes only, not by sutures, except in a few peculiarly specialized forms.