Ordovician life displays a notable advance over that of the Cambrian, becoming not only very much more varied and luxuriant, but also of a distinctly higher grade. During the long ages of the period also very decided progress was made, and when the Ordovician came to its close, all of the great types of marine invertebrates and most of their more important subdivisions had come into existence. In a general way the life of the Ordovician is an expansion of that of the Cambrian, though but little direct connection between the two can yet be traced, and evidently there were great migrations of marine animals from some region which cannot yet be identified. Several groups of invertebrates attained their culmination and began to decline in the Ordovician, becoming much less important in subsequent periods. Thus the Grap-tolites, the Cystoidean order of Echinoderms, the straight-shelled Cephalopods (orthoceratites) among Molluscs, and the Trilobites, were never so abundant and so varied as during this period.

Plate IV. - Ordovician Sponges, Corals, etc.

Fig. i, Zittelella typicalts Ulrich and Everett, x 1/2, Trenton. 2, Strobilospongia tuber. osa Beecher, x 1/2, Trenton. 3, Cyathophycus reticulatus Wale, x 1/2, Utica. 4, Recep~ taculites fungosus Hall, x 1/2, Trenton. 5, Petraia profunda Conrad, x 1/2 , Trenton. 5a, The same, top view. 5b, The same, vertical section. 6, Columnaria stellata Hall, x 1/2, Trenton. 7, Romingeria trentonensis Weller, x 1/2, Trenton. 8, Malocystites em~ monsi Hudson, x 2, Chazy. 9, Pleurocystites filitextus Bill., x 1, Trenton. 10, Lepi-dodiscus cincinnatiensis Hall, x 1/2, Richmond. 11, Glyptocrinus dyeri Meek, x 1/2, Richmond. 12, Blastoidocrinus carcharicedens, Bill , x 3/4, Chazy. 12 a, The same, basal view. 12 b, The same, side view. 13, Palceasterina stellata Bill., x 1/2, Trenton.

Ordovician Life. Plants

In America no plants above the grade of seaweeds and coralline Algae have been discovered, but in Europe a few of the higher Cryptogams are doubtfully reported. The flora of the Devonian, however, renders it highly probable that land plants were already well advanced in the Ordovician, and their remains may be discovered at any time. This must remain a matter of accident, for the known Ordovician rocks are almost all marine, which is not a favourable circumstance for the preservation of land plants. Such discoveries have, indeed, already been reported, but the evidence for them is not satisfactory.

Foraminifera and Radiolaria have been found in a few regions in great numbers, sufficient to prove that they were abundant in the Ordovician seas.

Ordovician Life. Spongida

Sponges are much more numerous and varied than in the Cambrian. Of course it is only those sponges with skeletons of lime or flint which can be well preserved in the fossil state, and of these the Ordovician has many (PI. IV, Figs. 1-4). The horny sponges, of which the common bath sponge is a familiar example, are necessarily much rarer and less satisfactory as fossils.

Ordovician Life. Ccelenterata

The Graptolites are very numerous and varied, wherever conditions are favourable to their preservation, as in fine-grained rocks with smooth bedding-planes. The Ordovician is the time of their culmination and is especially characterized by the double forms, with rows of cells on both sides of the stem Beekmantown. 6, Constellaria polystomella, Whitfield, x 1/2, Richmond. 7, Stictoporella cribrosa Ulrich, x 1/2 Trenton, 7a, The same, a fragment, x 9. 8. Stomatopora in-fiata Hall, on a brachiopod shell, x 1/2 Richmond. 8a, The same, a portion x 4. 9, Tre-matis ottawaensis Bill., + 1/2 Trenton. 10, Schizambon canadensis Ami., x 1, Utica. 11, Orthis tricenaria Conrad, x 1/2, Trenton. 11a, The same, lateral view. 12, Platy-strophia lynx Eichw., x 1/2, Trenton. 13. Hebertella sinuata Hall, x 1/2, Richmond. 13a, The same, inner side of ventral valve. 13b, The same, inner side of dorsal valve. 14, Dalmanella testudinaria Dalman, x 1/2, Trenton. 14a, The same, inner side of ventral valve. 14b, The same, inner side of dorsal valve. 15, Plectambonites sericeus Sowerby, X 1, Richmond. 16, Rafinesquina alternata Conrad x 1/2, Richmond. 16a, The same, longitudinal section. 17, Strophomena planumbonutn Hall, x 1/2, Richmond, 17a, The same, longitudinal section. 18, Zygospira modesta Say, X 1, Richmond. 18a, The same, from the side. 10, Rhynchotrema capax Conrad. x 1/2t Richmond. 19a, The same, ante, rior view. 20, Triplecia extans Emmons, x 1/2, Trenton. 20a, The same, anterior view.

Plate V. - Ordovician Graptolites, Brachiopods, etc.

Fig. i, Climacograptus bicornis Hall, x i, Utica. 2, Tetragraptus fruticosi4S Hall, x 1/2 Beekmantown. 3, Climacograptus pungens Ruedemann, x 5/2, Chazy. 4, Goniograp-tus postremus Rued., x 1/2, Beekmantown. 5, Phyllograptus augustifolius Hall, X 2, (see PL V, Figs. 1-5). So abundant are the Graptolites that in many parts of the system they are almost the only fossils, and are employed to divide the substages into zones. Graptolite zones, with the same or closely similar species, and in "the same order of succession, are found in Great Britain, the St. Lawrence and Champlain valleys, and in Australia. Hydroid corals, Stro-matocerium, are abundant, and form reefs in the Chazy and Black River limestones. The few and doubtful Cambrian Corals are succeeded by a considerable number of Ordovician genera and species. Like other Palaeozoic Corals, these are characteristically different from the reef-builders of the present day in showing a marked bilateral symmetry and having the septa arranged in multiples of four (Tetracoralla). Solitary cup-corals, like Strep-telasma and Petraia (IV, 5), and compound colonies, like Colum-naria (IV, 6), are examples of the range of differences among these early corals.