Indeed, in deposits belonging to the Lower Silurian and Devonian, in Russia, England, and North America, minute, slender, pointed horny bodies, bent like a hook, with sharp opposite margins, have been found and described under the name of _Conodonts_. More frequently they possess an elongated basal portion, in which there is generally a larger tooth with rows of similar but smaller denticles on one or both sides of the larger tooth, according as this is central or at one end of the base. In other examples there is no prominent central tooth, but a series of more or less similar teeth is implanted on a straight or curved base. Modifications of these arrangements are very numerous, and many Palæontologists entertain still doubts whether the origin of these remains is not rather from Annelids and Mollusks than from Fishes. [See G. J. Hinde, in “Quarterly Journal of the Geological Society,” 1879.] The first undeniable evidence of a fish, or, indeed, of a vertebrate animal, occurs in the _Upper Silurian_ Rocks, in a bone-bed of the Downton sandstone, near Ludlow. It consists of compressed, slightly curved, ribbed spines, of less than two inches in length (_Onchus_); of small shagreen-scales (_Thelodus_); the fragment of a jaw-like bar with pluricuspid teeth (_Plectrodus_); the cephalic bucklers of what seems to be a species of _Pteraspis_; and, finally, the coprolitic bodies of phosphate and carbonate of lime, including recognisable remains of the Mollusks and Crinoids inhabiting the same waters. But no vertebra or other part of the skeleton has been found. The spines and scales seem to have belonged to the same kind of fish, which probably was a Plagiostome. It is quite uncertain whether or not the jaw (if it be the jaw of a fish[16]) belonged to the buckler-bearing _Pteraspis_, the position of which among Ganoids, with which it is generally associated, is open to doubt. No detached undoubted tooth of a Plagiostome or Ganoid scale has been discovered in the Ludlow deposits: but so much is certain that those earliest remains in Palæozoic rocks belonged to fishes closely allied to forms occurring in greater abundance in the succeeding formation, the Devonian, where they are associated with undoubted Palæichthyes, Plagiostomes as well as Ganoids. * * * * * These fish-remains of the _Devonian_ or _Old Red Sandstone_, can be determined with greater certainty. They consist of spines or the so-called _Ichthyodorulites_, which show sufficiently distinctive characters to be referred to several genera, one of them, _Onchus_, still surviving from the Silurian epoch. All these spines are believed to be those of Chondropterygians, to which order some pluricuspid teeth (_Cladodus_) from the Old Red Sandstone in the vicinity of St. Petersburg have been referred likewise.