Page:The American Cyclopædia (1879) Volume XII.djvu/825

Rh thinner beds. M. d'Archiac and M. de Verneuil have observed that the persistence of species is connected or corresponds with the extent of their geographical distribution. "The species which are found at the same time in many points and in far separated countries are almost always those which have lived during the formation of several successive epochs. ("Transactions of the Geological Society," 2d series, vol. vi., p. 335.) This may be regarded as true in general, and for the same reason they may have continued their existence; but we have an exception worthy of note in the brachiopodous genus tropidoleptus, which first appears in the Hamilton group, and occurs in this group from near the Hudson to beyond the Mississippi, and is found likewise in Europe, while neither the genus nor any form of the type is known in any later formation. With a few individual exceptions, the differences which exist between the lost or fossil faunas and the existing animals are the greater according as the former are more ancient. The recent forms are more varied; the diversity of animal organization has augmented in the lapse of time. But this does not warrant the inference that because the most ancient faunas are composed of animals of lower organization, the degree of perfection, as a rule, increases as we approach the more recent epochs. We find the more and the less perfect, or the lower and the more highly organized, living contemporaneously in the later geological periods, and each epoch presenting a fauna and a flora adapted to the existing physical conditions. The order of appearance of different types has been shown to represent the phases of embryonic development. The comparison of faunas of different epochs shows that the temperature has varied upon the surface of the globe, and that the areas of varying temperature have not corresponded with the present, but have resulted from physical causes existing at the time, producing wider areas of uniform conditions; and to this cause may be attributed the fact that the species of the ancient formations have had a wider geographical distribution than those of the present time. In conclusion we find that fossil animals were constructed upon the same plan as the existing animals; and that the same zoological principles are applicable in their study and classification.

Order Of fossil sponges, palæospongia and acanthospongia and other forms occur in the Cambrian; and astyloapongia, astræospongia, palæomanon, &c., in the Silurian. Stromatopora, which is placed in this class by some naturalists, occurs abundantly at the base of the Trenton, and in the Niagara group; and near the base of the lower Helderberg it forms a stratum four feet thick and extending over many miles. It is likewise abundant in the upper Helderberg and in the calcareous portion of the Chemung group. It is abundant in the Wenlock limestone of England. Other genera of sponges occur in carboniferous, Permian, triassic, Jurassic, cretaceous, and tertiary strata.

Order. Organisms of this order are usually minute or microscopic. Some forms are known in the Cambrian and Silurian rocks of Europe. Ehrenberg has described species of textularia, rotalia, and guttulina from the greensands of the obolus or ungulite grits of Russia, which probably hold the same position as our Potsdam sandstone In the carboniferous limestones of Ohio, Indiana, Illinois, Iowa Missouri and Kansas, rotalia, fusulina, &c., are Abundant, and probably other genera. The last named fossil forms layers of considerable thickness, known as fusulina limestone. The foraminifera are abundant in the Jurassic period, but acquire their greatest development in the latter part of the cretaceous and eocene tertiary, in the nummulites, orbitoides, orbitolina, &c. The nummulitic limestones are found in France and southern Europe, northern Africa, and India, in the United States, and in the island of Jamaica. The most common form is seen in the limestone of the great pyramids of Egypt. Receptaculites, which is regarded by some naturalists as a gigantic foraminifer, is known in the Trenton group and is abundant in the upper member of the formation, one species being a foot in diameter. It occurs also in the Niagara, lower Helderberg, and upper Helderberg formations. Ihe eozoön of the Laurentian has been referred to this order.

Class. Graptolitidæ. The graptolites are characteristic Cambrian and lower Silurian fossils, and most abundant on the confines of these two systems. Species of the several graptolitic genera range from the Potsdam sandstone to the Clinton group inclusive. Dictyonema, a graptolitic genus, is known from the base of the Trenton to the Hamilton group, or middle Devonian. Oldhamia, the oldest known fossil of the European Cambrian, is probably a graptolitic genus.

Class (corals, madrepores). Of corals, the cyathophylloid type (order zoantharia rugosa) begin their existence in the Cambrian, and are known from the base of the Trenton group, through all the formations, to the close of the palæozoic era, acquiring their greatest development in the Devonian (corniferous and Hamilton). Corals of the same form, with different internal structure, known as the order zoantharia aporosa, characterize the formations from the trias to the present time. The latter are known as the neozoic, and the former as the palæozoic type. Corals of the madrepore tribe (zoantharia tabulata), as columnaria, favistella, favosites, &c., begin their existence about the same time as those of the cyathophylloid type, acquiring their greatest development in the Devonian, and disappearing at the end of the carboniferous period.

Class. 1. Crinoidea (encrinites, stone lilies). 2. Cystidea (cystideans). 3. Elastoidea (pentremites). 4. Asteriadæ (sea stars, star fish). 5. Ophiuridæ (sand stars, serpent stars). 6. Echinidæ and palechinidæ (sea urchins). 7. Holothuridæ (sea cucumbers). The encrinites are the earliest type of this class, and appear in the Cambrian system of Europe, and in rocks of the same age in America. The fossils of this family first become numerous in the Trenton period, and are abundant in the Niagara and lower Helderberg. They are usually not abundant in the Devonian, though the Hamilton group has yielded a considerable number of species; but they become extremely abundant in the carboniferous, with the most extravagant forms in the lias, and from that time decline to the present epoch. The cystideans begin their existence at the same time as the encrinites, and are common in the Trenton and Niagara periods, gradually disappearing with the close of the Silurian age. The pentremites proper are of rare occurrence in the Silurian period, but become common in the Devonian, and reach their greatest development in the carboniferous, where they disappear. The star fishes are first known in the Trenton period, and continue with increasing numbers through the Devonian and carboniferous epoch, and they occur in varied forms through the Jurassic, cretaceous, and tertiary, to the present time. The ophiuroids (sand stars or serpent stars) begin their existence in the lower Silurian, and extend through the Devonian and carboniferous. They are unknown in the Permian and triassic, but occur in the Jurassic, cretaceous, and tertiary, and are more numerous in the modern seas. The echinoids, of the type of palechinus or eocidaris, commence in the Devonian period and continue through the carboniferous. The true echinidæ and cidaridæ begin in the