Clean and remove some of the caudal vertebrae from the region just back of the cloaca. In general they have nearly the same structure and relations as the thoracic vertebrae, but have no transverse processes and the plates of the neural arches are not so distinct. There is also added a ventral arch similar in form to the neural arch. This is the _haemal arch_, in which lie the caudal aorta and vein. Its roof is the surface of the centrum, the sides are formed by pairs of plates which correspond in number to the centra, and unite with each other ventrally. Between the successive plates are openings for the passage of branches of the artery and vein. In this region foramina for the roots of the spinal nerves are found only in every other pair of neural and intercalary plates. Toward the tip of the vertebral column the relation of the neural and intercalary plates to the centra becomes very irregular. In Eugaleus the roof of the neural arch is formed by a row of small, diamond-shaped plates which fit in between the other two sets. As these plates correspond morphologically to the neural spines of higher vertebrates, they may receive that name here. It is probable that the dorsal portion of the arch in Squalus is composed of similar neural spine elements which have become fused with the neural and intercalary plates of each side. SKULL. The skull is entirely cartilaginous, and comprises three principal divisions: (1) the _cranium_, an undivided mass of cartilage lodging the brain and the organs of smell, sight, and hearing; (2) the _jaws_; (3) the _visceral arches_, or skeletons of the gill arches. (1) The cranium. A blunt prolongation of the anterior extremity of the cranium forms the _rostrum_, which supports the soft tissues of the snout. At each side of the base of the rostrum the cranium widens abruptly. On the anterior face of the widened portion and below the posterior angles of the rostrum is a pair of protruding _olfactory capsules_, in which the olfactory sacs are enclosed. An oval aperture in the posterior wall of each capsule opens into the braincase and permits the passage of the olfactory nerve through the cranium. Back of the olfactory capsules are large lateral cavities, the _orbits_. The dorsal edge of the orbit makes an overhanging ledge, known as the _supra-orbital crest_. The projecting anterior and posterior angles of the orbit are distinguished as the prae- and postorbital processes. The portions of the cranium back of the orbit and at the sides of the braincase form large lateral projections (_auditory capsules_) containing the organs of hearing. At the center of the nearly vertical posterior surface of the cranium is a large opening, the _foramen magnum_, through which the spinal cord passes. At either side of and below the foramen magnum is a smooth articulatory surface (_occipital condyle_) articulating with the centrum of the first vertebra.