VERTEBRATA.
There is an enormous series of subaqueous sediment, originally composed of mud, sand, or pebbles, the successive bottoms of a former sea, derived from pre-existing rocks, which has not undergone any change from heat, and in which no trace of organic life has yet been detected. These non fossiliferous, non-crystalline, sedimentary beds form, in all countries where they have yet been examined, the base-rocks on which the Cambrian or oldest Silurian strata rest.
Whether they be significative of ocean abysses never reached by the remains of coeval living beings, or whether they truly indicate the period antecedent to the beginning of life on this planet, are questions of the deepest significance, and demanding much farther observation before they can be authoritatively answered.
It has been shown that every type of invertebrate animal is represented in the superimposed stratified deposits called Cambrian and lower Silurian.
An important work,* embodying the labours of the accomplished naturalist and acute observer, Dr. Christian H. Pander, has recently been published by the Russian govern ment, descriptive of the fossil fishes of the Silurian formations of that empire. Of some hundred fossils described and beautifully figured in this work, and referred to different genera and species of fishes, from lower Silurian rocks, the writer, after the closest comparison and consideration of the evidence, is disposed to regard only those referred by Pander to the genera Ctenognathus, Cordylodus, and Gmathodus, as having any probable claims to vertebrate rank ; and to this admission must be appended the remark, that the parts referred to jaws and teeth may be but remains of the dentated claws of Crwstacea. With regard to the fossils called " Conodonts," on which the main part of M. Pander's evidence of lower Silurian fishes rests, the following remarks, penned after microscopic examination of original specimens, are applicable to them.
Minute, glistening, slender, conical bodies, hollow at the base, pointed at the end, more or less bent, with sharp opposite margins, might well be lingual teeth of Gastropods, acetabular hooklets of Cephalopods, or teeth of cartilaginous fishes.
Against the latter determination is the minute size of the " Conodont " bodies. Their basal cavity doubtless contained a formative pulp, but the proof that the product of such pulp was " dentine " is wanting : the observed structure of the hooklet presents concentric conical lamellae of a dense structureless substance, containing minute nuclei or cells.
In some specimens the base is abruptly produced and divided from the body of the hooklet by a constriction—a form unknown in the teeth of any fishes, but presented by certain lingual teeth of Gastropods—e. g., the lateral teeth of Sparalla. In other Conodonts the elongated base is denticulate or serrate, as in the lateral teeth of Buccinwm and Chrysodomus. It is improbable, however, that they belong to any conchiferous toothed Mollusk, the shells of such being wanting in the deposit where the Conodonts are most abundant.
The more minute booklets have a yellowish, transparent, horny appearance ; the larger, perhaps older ones, present a harder whitish appearance. Their analysis by Pander yielded "carbonate of lime," carbonic acid being evolved by application of dilute nitric acid, and oxalic acid producing an obvious pre cipitate. Some English analysts have believed that the Cono donts yielded a trace of phosphate of lime.
The detached condition of the hooklets, and the integrity of the thin border of the basal pulp-cavity, indicate that they have not been broken away from any of those kinds of attachment to a bone which the minute villiform teeth of osseous fishes would show signs of. The Conodonts have been supported upon a soft substance, such as the skin of a mollusk or worm, the mucous membrane of a mouth or throat„ or the covering of a proboscis ; but to select the teeth of cyclostomous or plagiostomous fishes as the exclusive illus tration of the above condition, would be to take a partial and limited view of the subject.