1. horny teeth. 2. labial cartilage. 3. anterior dorsal cartilage. 4. posterior dorsal cartilage. 5. nasal capsule. 6. auditory capsule. 7. dorsal portion of trabeculae. 8. lateral distal mandibular. 9. lingual cartilage. 10. branchial basket. 11. cartilaginous cup supporting pericardium. 12. sheath of notochord. 13. neural plate.] In Myxinoids and larval lampreys, the notochord is enclosed in a thick chordal sheath, in connection with which in the tail region there occur cartilaginous pieces forming neural arch elements. In the trunk region, however, no cartilage occurs in connection with the spinal column, the only cartilage present being that forming the radiale of the dorsal fin. On the other hand in most species of lamprey (_Petromyzon_) cartilaginous pieces forming imperfect neural arches (fig. 4, B, 13) are found lying in the tough skeletogenous layer dorsal to the notochord, and extending throughout the whole length of the trunk and tail. Two of these pieces, which are probably homologous with the neural plates (see p. 72) of Elasmobranchs, occur to each _neuromere_, or segment as determined by the spinal nerves. The dorsal and caudal fins are supported by paired cartilaginous radiale which are connected proximally with the skeletogenous layer. THE SKULL. In Myxinoids the cranium is a mere cartilaginous floor without side walls or roof, and the trabeculae[27] end without growing forwards into cornua. In Lampreys the trabeculae grow forwards and send up plates of cartilage which meet above (fig. 4, 7) and form side walls and a roof for part of the brain case. In Lampreys a labial suctorial apparatus is well developed, including a large ring-like piece of cartilage (fig. 4, 2) which supports the oral funnel and bears a large armament of horny teeth. In Myxinoids on the other hand the labial skeleton is small and consists merely of barbels round the mouth. The olfactory organ of Myxinoids has a very curious skeleton. It is covered with a kind of grating of cartilage which is prolonged in front into a tube composed of a series of imperfect cartilaginous rings. In Lampreys the olfactory organ opens merely by a short membranous passage. In correlation with the small development of the labial suctorial apparatus in Myxinoids the lingual apparatus is very greatly developed. The tongue in _Myxine_ has been said to 'dominate the whole body' (Parker). It is supported by a great median cartilaginous bar which when followed forwards first becomes bifid and still further forwards becomes four-cleft. The horny teeth in Myxinoids are chiefly borne on the very large supralingual apparatus. They form a double series arranged in the form of an arch. In _Myxine_ there are seven large teeth and nine small ones on each side. In _Bdellostoma_ the teeth of the two rows are more equal in size. In _Bdellostoma_ and _Myxine_ it has been shown that imperfect calcified teeth occur below the horny teeth.