26 



like projection, the transverse process. Attached to the extremity 

 of this is a slender cartilaginous rib. 



Each neural arch is made up of two distinguishable sets of 

 plates. The first consists of a pair of broad neural plates extend- 

 ing upward from each side of the centrum and uniting with each 

 other dorsally. Between the neural plates of two successive verte- 

 brae is a pair (one on each side) of intercalary plates which also 

 unite over the neural canal. The intercalary plates are over the 

 joint between the centra. Neural and intercalary plates together 

 make the lateral and dorsal walls of the neural canal. The rela- 

 tions of these plates can sometimes be seen best when the neural 

 arch is cleaned, then cut away from the centrum, and looked through 

 toward the light. 



In the lower part of each neural plate is a small foramen which 

 allows the passage of the ventral root of the spinal nerve. A fora- 

 men for .the dorsal root is found at about the middle of the inter- 

 calary plate. 



Clean and remove some of the caudal vertebrae from the region 

 just back of the cloaca. In general they have nearly the same struc- 

 ture 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, dia- 

 mond-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 car- 

 tilage lodging the brain and the organs of smell, sight, and hear- 

 ing; (2) the jaws; (3) the visceral arches, or skeletons of the gill- 

 arches. 



(1) The cranium. A blunt prolongation of the anterior ex- 

 tremity 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 en- 

 closed. 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 pos- 

 terior angles of the orbit are distinguished as the prae- and post- 

 orbital processes. 



The portions of the cranium back of the orbit and at the sides 



