In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed, but not excessively so. It consists of a long median cartilaginous bar which ends in front with a semicircular piece of cartilage supporting the median part of the tongue. In both Myxinoids and Lampreys there is a complicated branchial basket apparatus, but while in Myxinoids the basket apparatus is interbranchial, formed deep within the head near the hypoblastic lining of the throat, in Lampreys it is extra-branchial and formed outside the head cavities (fig. 4, 10). The two sides of the basket apparatus in _Myxine_ are not symmetrical. In the interbranchial basket apparatus of Myxinoids the hyoid and first and second branchial arches can be recognised. Traces of the interbranchial skeleton of Myxinoids can be detected in Lampreys, and similarly in Myxinoids, there are indications of the extra-branchial skeleton of _Petromyzon_. The branchial basket in Lampreys forms at its posterior end a kind of cup which supports the pericardium (fig. 4, 11). A remarkable Cyclostome named _Palaeospondylus_[28] has recently been described from the Scottish Old Red Sandstone. It differs however from all living Cyclostomes, in having a spinal column formed of distinct vertebrae with well-developed neural arches. The caudal fin is well developed and the dorsal radiale are forked as in lampreys. The skull is well calcified and the auditory capsules are specially large. The mouth is very similar to that of lampreys, being circular and without jaws; it is provided with barbels or cirri. There is no trace of limbs and the average length is only about 1-1-1/2 inches. FOOTNOTES: [24] See W.K. Parker On the skeleton of the Marsipobranch fishes, _Phil. Trans._ 1883, London. [25] See A. Smith Woodward, _Catalogue of Fossil Fish in the British Museum_, Part II., 1891. A. Smith Woodward, _Nat. Sci._ vol. I. 1892, p. 596. [26] See R.H. Traquair, _Ann. Nat. Hist._, ser. 6, vol. II. 1888, p. 485. [27] See p. 17. [28] R.H. Traquair, _Ann. Nat. Hist._ vol. VI. 1890, p. 485; _P. Phys. Soc. Edinb._ vol. XII. 1892-93, pp. 87-94, and 312-320. A. Smith Woodward, _Nat. Sci._ vol. III. p. 128, 1893. CHAPTER V. (II.) GNATHOSTOMATA. THE mouth is supported by definite jaws. ICHTHYOPSIDA. The epiblastic exoskeleton is generally unimportant, the mesoblastic exoskeleton is usually well developed. The notochord with its membranous sheath (1) may remain unmodified, or (2) may be replaced by bone or cartilage derived from the skeletogenous layer, or (3) may be calcified to a varying extent. The first vertebra is not homologous throughout the whole series and so is not strictly comparable to the atlas of Sauropsids and Mammals.