Confessedly the highest style of animal is that which possesses a skull and backbone, with brain and nerve system to match, and which embodies the general plan of structure employed in man himself. Yet among the fishes, which constitute the lowest manifestation of this type, are some so rudimentary that the brain is scarcely developed, and the skeleton is merely a cord of gristle. These are represented in the modern world only by the Lancelot,[33] a creature which has sometimes been mistaken for a worm, and by a slightly more advanced type, that of the Lampreys.[34] In these animals the Vertebrates make the nearest approach to the lower domains of the animal kingdom, collectively known as Invertebrates. We should naturally expect that since the vertebrates succeed the inferior animals in time, their lower types should appear first, and that these should be aquatic rather than terrestrial. On the other hand, as the oldest fishes that are certainly known are strongly protected with bony armour, and had to contend against formidable Crustaceans and Cuttles, we might suppose that the Lancelot and the Lampreys are rather degraded types belonging to the modern period, than the true precursors of the other fishes. [Illustration: FIG. 102.—Siluro-Cambrian Conodonts. Magnified.—After Pander.] But if fishes like the Lancelot preceded all others, we may never find in a fossil state any traces of their soft and perishable bodies; and even the Lampreys have no hard parts except small horny teeth, which might easily escape observation. But palæontologists have sharp eyes, and it has not escaped them that certain microscopic tooth-like bodies are somewhat widely distributed in the older rocks. In Russia, Pander has found in the Upper Cambrian and Lower Silurian, and also in the Devonian and Carboniferous, minute conical and comb-like teeth, to which he has given the name of _Conodonts_ (Fig. 102), and which he supposes to be the teeth of ancient Lampreys. Similar teeth have been found by Moore and others in the Carboniferous of England, and by Newberry in Carboniferous shales in Ohio. In point of form, these bodies certainly resemble the teeth of the humble fishes to which they have been referred. In the case of the Carboniferous specimens from Ohio—the only ones I have had an opportunity to examinethe material is calcium phosphate, and the structures are more like those of teeth of Sharks than of Lampreys, so that there can be no doubt that they are really teeth of fishes, and probably of fishes of somewhat higher grade than the Lampreys.[35] The Cambrian and Silurian specimens are said to be composed of calcium carbonate, which would render it more probable that, as has been suggested by Prof. Owen, they may have been teeth of some species of Sea-snail destitute of shell. It is, however, possible that they may have originally been horny, and that the animal matter has been replaced by carbonate of lime. Rohon and Zittel have recently shown that