1. cartilaginous part of the7. pre-opercular (squamosal).
quadrate with which the8. second rib.
mandible articulates.9. first rib.
2. scleroparietal.10. vomerine tooth.
3. frontal.11. palato-pterygoid tooth.
4. ethmoid.12. palato-pterygoid.
5. nares.13. parasphenoid.
6. orbit.14. interopercular.

Ribs.

As has been already mentioned (p. 24), although ribs commonly appear to be the cut-off ends of the transverse processes, they are really elements derived from the ventral or haemal arch.

In Elasmobranchii and other cartilaginous fish they have the form of small cartilaginous structures imperfectly separated from the diverging halves of the ventral arch, and are often absent.

In Teleostei and bony Ganoids they often have different attachments in different parts of the body. In the tail region they are not differentiated from the two halves of the ventral arch, which meet in the middle line, and are prolonged into a haemal spine. In the posterior trunk region they sometimes form distinct processes diverging from the two halves of the ventral arch; while further forward they may shift their attachment so as to arise from the dorsal side of the two halves of the ventral arch and at some distance from their ends, which now diverge as ventri-lateral processes.

Appendicular Skeleton.

Pectoral girdle.

The simplest type of pectoral girdle is found in Elasmobranchs. It is entirely cartilaginous and consists of a curved ventrally-placed rod, ending dorsally in two horn-like scapular processes which are sometimes attached to the cranium or vertebral column. In Rays the shoulder girdle is very large, and has a distinct suprascapular portion forming a broad plate attached to the neural spines of the vertebrae. There is often a cup-like glenoid cavity for the articulation of the limb; this cavity is specially large in Rays and is much pierced by holes. In Dipnoi the cartilaginous girdle still occurs, but on it there is a deposit of membrane bone forming the clavicle, infraclavicle, and supra-clavicle. These bones, which with the exception of the clavicle, are unknown in higher vertebrates, are better developed in Ganoids, and best of all in Teleosteans. They are connected by the supratemporal with the epi-otic and opisthotic regions of the cranium. Owing to this development of dermal bone, the original cartilaginous arch becomes much reduced, but ossifications representing the scapula and coracoid occur in bony Ganoids and Teleosteans.

Pelvic girdle.

In Elasmobranchs the pelvic girdle consists of a short ventral rod of cartilage representing the ischium and pubis, which does not send up dorsal iliac processes. In Chimaera the pelvic girdle has a flattened pointed iliac portion, and ventrally an unpaired movable cartilaginous plate which bears hooks and is supposed to be copulatory in function. Claspers of the usual type are present as well. The Dipnoi have a primitive kind of pelvis in the form of a cartilaginous plate lying in the mid-ventral line and drawn out into three horns anteriorly. In Ganoids the pelvis has almost entirely disappeared, though small cartilaginous vestiges of it remain in Polypterus. In Teleosteans even these vestiges are gone, and in these fish and Ganoids the place of the pelvis is taken by the enlarged basi-pterygia (meta-pterygia) of the fins.