The first few vertebrae differ from the others in having very short centra and no ventri-lateral processes. The first vertebra comes into very close relation to the posterior part of the skull, articulating with the exoccipitals. In the next few vertebrae the centra gradually lengthen, and at the fourth or fifth vertebra the ventri-lateral processes appear and gradually increase in size as followed back. They likewise gradually come to arise at a lower level on the centrum, and also become more and more downwardly directed, till at the last trunk vertebra they nearly meet. The =neural spines= of the anterior trunk vertebrae are much longer than those of the posterior ones, that of the first vertebra being the largest and longest of all, and articulating with the skull. The spinal nerves pass out through wide notches or spaces between the successive neural arches. =Caudal vertebrae.= The caudal vertebrae are about thirty-five in number, each consists of a centrum with a slender backwardly-directed dorsal or neural arch, similar to those of the posterior trunk vertebrae. The two halves of the ventral or haemal arch however do not form outwardly-directed ventri-lateral processes, but arise on the ventral surface of the centrum, and passing downwards meet and enclose a space; they thus form a complete canal, and are prolonged into a backwardly-directed ventral or =haemal spine=. The anterior haemal arches are much larger than the corresponding neural arches, but when followed back they gradually decrease in size, till at about the twenty-fourth caudal vertebra they are nearly as small as the neural arches. The last caudal vertebra is succeeded by a much flattened =hypural= bone or =urostyle=, which together with the posterior neural and haemal spines supports the tail-fin. B. THE RIBS. The =ribs= are slender, more or less cylindrical bones attached to the poster-dorsal faces of the ventri-lateral processes of all the trunk vertebrae except the first and second. The earlier ones are thicker and more curved; the later ones thinner and more nearly straight. The ribs are homologous with the distal parts of the haemal arches of the caudal vertebrae. Associated with the ribs are a second series of rib-like bones, the =intermuscular bones=. These are slender, curved bones which arise from the ribs or from the ventri-lateral processes at a distance of about an inch from the centra, and curve upwards, outwards and backwards. In the anterior region where the ventri-lateral processes are short they arise from the ribs, further back they arise from the ventri-lateral processes. C. THE UNPAIRED OR MEDIAN FINS. These are six in number, three being =dorsal=, one =caudal= and two =anal=.