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lias and continue to the present time. The holothurians, which are soft, cylindrical bodies, are found in the lias, being recognized from certain minute calcareous parts known to belong to this order of echinoderms.

Class (conchifera, bivalve shells). The fossils of this class of shells first appear, as representatives of the families nuculidæ and arcadæ, in the Cambrian period, and are known in the Quebec group and Trenton limestone, in the genera tellinomya, nuculites, and palæarca; and the aviculidcæ and mytilidæ are represented in the Trenton period. They gradually increase in numbers through the Silurian strata, until in the Devonian (the Hamilton and Chemung groups) these fossils, in the genera aviculopecten, pterinea, nucula, nuculites, grammysia, &c., are sometimes more numerous than the brachiopoda. In later times the shells of this class acquire a much greater importance, and largely preponderate over the brachiopoda.

Class (univalve shells). The fossils of this class began their existence in the earliest geological periods, and we recognize them in the Potsdam sandstone of America and the Cambrian of Europe. The earlier forms belong to the families ''turbinidæ, haliotidæ, calyptræidæ, bellerophontidæ, atlantidæ, &c. Maclurea, ophileta, and euomphalus'' occur in the calcifarous sandstone.

Class. The fossils of this class commence in the lowest fossiliferous strata. The genus theca occurs in the Potsdam sandstone. Pterotheca and conularia are found in the Trenton limestone, and the latter continues through the Silurian, Devonian, and carboniferous periods.

Class. The fossils of this class have existed in all geological ages, and appear in varied and conspicuous forms. The oldest known in the Cambrian system are orthoceratites, of which we have great numbers in the Black river and Trenton limestones. Associated with these are lituites and cyrtoceras, which continue to the Devonian period. The genus goniatites begins in the Devonian and continues through the carboniferous. In the triassic begin the genera ceratites and ammonites; the latter appear in great numbers and variety of form in the Jurassic period, and still continue, together with scaphites, turrilites, and baculites, in the cretaceous system, beyond which these forms do not occur. The genus nautilus appears in the Silurian, and continues through all the geological periods to the present time.

Class or. These are bodies consisting mainly of branching or reticulated calcareous fronds, rising from a root, or of expansions adhering to other bodies, minutely celluliferous. The most common palæozoic forms Are fenestella, retepora, polypora, &c. In America these fossils reach their maximum development in the carboniferous period, where the spreading fronds are sustained by a strong central axis, upon which they grow in a spiral arrangement, as in the genus Archimedes.

Class. The fossils of this class are everywhere abundant, and are the best guides in the study of the strata throughout the palæozoic period. The linguloid type, in the genera lingulella and lingulepis, begins in the lowest known fossiliferous formation, and continues through every geological epoch to the present time. The genus obolus characterizes the oldest Cambrian beds of Europe and the Potsdam of America. The orthidcæ, in the genera orthis and orthisina, begin in the Cambrian, become more abundant in the Silurian, and, gradually lessening, die out at the end of the carboniferous period. The same is true of the strophomenidæ, represented in the genera strophomena, leptæna, strophodonta, and streptorhynchus. The rhynchonellidæ begin in the Chazy limestone, and continue throughout all the geological periods. They are especially abundant in the upper Silurian, Devonian, and carboniferous periods. The spiriferidæ, in zygospira, begin in the Trenton formation, and characterize the Clinton and Niagara groups in the genera spirifera, cyrtia, cyrtæna, and meristella. The genus athyris begins in the Devonian, and the family dies out in the genus spiriferina in the Jurassic period. The pentameroid forms begin in the Trenton and disappear in the Devonian. The family productidæ appears in the genus chonetes at the period of the Clinton group, and continues through the carboniferous. Productus (productella) begins in the upper Helderberg, continues through the carboniferous, and disappears in the Permian. The discinidæ, in the genera discina, trematis, &c., begin their existence in the Trenton and Hudson river periods, and are represented by discina, orbiculoidea, and allied forms, through the Devonian and carboniferous periods, and to the end of the geological series. In like manner the craniadæ begin as low as the Trenton and extend through the entire series. The terebratulidæ are first represented in the upper Silurian of the lower Helderberg, and are thence known in the genera cryptonella, centronella, terebratula, terebratella, Waldheimia, &c., to the end of the series.

Class (ascidea). The forms of this class have no solid shell, the soft parts being protected by an elastic gelatinous covering having two orifices. The soft character of these organisms precludes their preservation in the rocky strata.

Class. The tracks of animals of this class (or perhaps of crustaceans) are found in the Potsdam sandstone of America, and in the Cambrian rocks of Europe. The order tubicola is represented in the lower strata by serpuloid forms. Arenicolites, or worm burrows, are common in the oldest fossiliferous rocks; so that this class of organisms has existed from the most ancient times.

Class. The entpmostraca in the order trilpbita is represented in numerous genera, beginning in the lower Cambrian rocks, extending through the Silurian and Devonian; gradually lessening in the latter, and dying out entirely in the carboniferous period. The order merostomata, represented in the genera eurypterus, pterygotus, slimonia, dolichopterus, stylonema, and arthropleura, begin their existence in the middle Silurian, and extend in some variety of form to the close of the coal measures. The phyllopoda commence near the base of the Silurian proper, and appear in the palæozoic rocks in the genera ceratiocaris, dithyrocaris, and others. The ostracoda or cyproidea, in the form of small bivalved crustaceans of the genera primitia, leperditia, cytherina, Beyrichia, bolbozoa, aristozoa, &c., occur in the lowest fossiliferous rocks, and continue through the palæozoic age; these are followed in later periods by cythera, cypris, cyprella, cypridella, &c. The podophthalmia or decapoda, including the crabs, lobsters, and their allies, commence their existence after the palæozoic era, and continue through all the formations to the present time. The cirripedia begin as low as the Trenton limestone, in the form of plumulites or turrilepas; continuing but sparsely below the carboniferous, where they become more common, and increase in number of forms through the succeeding epochs to the present.

Class. Fossil spiders are found in the carboniferous strata, in the Solenhofen slates of the Jurassic, and in the tertiary.

Class. Centipedes and millipedes begin their existence in the carboniferous period.

Class. Fossil remains of insects have been found in the lower coal measures and in the Devonian, and more abundantly in the lias limestone of Europe. In some portions of the tertiary formation of the western territories their remains are abundant, and also in the tertiary of Europe.

Class (fishes). The ganoids, an order covered with bony plates, like the garpike and sturgeon, are among the earliest known forms of fishes. The genera coccosteus, pterichthys, onychodus, macropetalichthys, &c., occur in the upper Helderberg, lower Devonian, and subsequent formations. The genera cephalaspis and holoptychius are known in the old red sandstone of Europe, and the remains of the latter genus abound in the Catskill formation of New York and Pennsylvania. The genera eurylepis, cœlacanthus, and others are known in the coal measures of Ohio. Of the selachians, or fishes with cartilaginous skeletons, the cestraclonts begin their existence in the age of the upper Helderberg, and continue through all the formations to the present time. The hybodonts first appear in the lower carboniferous, and continue through succeeding formations. The raiidæ (rays) begin their existence in the carboniferous, and are known in the lias and eocene formations, and in the present fauna. The squalodonts, or true sharks, first appear in the cretaceous period, and continue to the present time. The teliostei, or bony fishes, such as perch, cod, salmon, &c., commenced in the trias. Some forms occur in the middle and upper Jurassic, but are not common below the cretaceous period. No fossil fishes are known in the Silurian of America; and it may be considered doubtful if the European species cited as Silurian are really older than our own.

Class (batrachians). The order labyrinthodonta is represented in the carboniferous period of Europe and America. The apateon and archegosaurus are European