Section 9. The internal skeleton, as we have said, is entirely cartilaginous, and only those parts which are pre-formed in cartilage in the skeletons of the higher types are represented here. The spinal column consists of two types of vertebrae, the trunk, bearing short, distinct, horizontally-projecting ribs (r.), and the caudal. The diagrams of Figure 5 [(Sheet 18)] are to illustrate the structure of the centrum of a dog-fish vertebra; C is a side view, D a horizontal median section, A and B are transverse sections at the points indicated by -B and A- [A and B] respectively in Figure C. -(By an unfortunate slip of the pen in the figure, A was substituted for B; section A corresponds to line B, and vice versa.)- The vertebrae are hollowed out both anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord runs through the entire length of the vertebral column, being constricted at the centres of the centra, and dilated between them. The neural arch above the centrum, and containing the spinal cord, is made up of neural plates (n.p.), and interneural plates (i.n.p.), completed above by a median neural spine (n.s.). In the caudal region, instead of ribs projecting outwardly, there are haemal processes, inclined downwards and meeting below, forming an arch, the haemal arch, containing the caudal artery and vein-- the vein ventral to the artery-- and resembling the neural arch, which contains the spinal cord above, in shape and size. Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have only a very vague resemblance to the corresponding structures in the rabbit. The girdle (g.) is a transverse bar lying ventral to the pericardial wall, and sending up a portion (sc.), dorsal to the attachment of the limb, which answers to the scapula and supra-scapula of the forms above the fish. Three main cartilages, named respectively the pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the limb. With these, smaller cartilaginous plates, rods, and nodules articulate, and form a flattened skeletal support for the fin. Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are similar in structure, but the pro-pterygium and meso-pterygium are absent, and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium. In the male, but not in the female, the pelvic fins are united behind the cloaca, and there are two stiff grooved copulatory organs, the claspers (cl. in Figure 1), which have a cartilaginous support (cl.c.). These claspers form the readiest means of determining the sex of a specimen before dissection.