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.
[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.
[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.
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.
The centra of the vertebrae have no epiphyses. The skull may be (_a_) incomplete and membranous, or (_b_) more or less cartilaginous, or (_c_) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (_a_) cartilaginous, (_b_) partially ossified, or (_c_) membrane bones may be developed in connection with it,--if so, there is usually more than one membrane bone developed in connection with each half.