The Echinodermata have greatly increased in importance, and all the main subdivisions of the group are represented, all of which, except the Cystoids, first appear in the Ordovician. The Cystoidea, which we have already found in the Cambrian, attain their greatest development in the Ordovician. In these curious animals the body is either irregularly shaped, or symmetrical, with a short, tapering stem, by which the animal was attached to the sea-floor, and weakly developed arms. The body, or calyx, is made up of a number of calcareous plates; when these plates are very numerous, they are of irregular size and arrangement (IV, 8, 10), while the forms with few plates have them of a definite number, size, and shape (IV, 9). Some of the more regular Cystoidea have much resemblance to the true Crinoids. The latter are not rare, though less abundant than they afterward became. These aninials (IV, 11) have a symmetrical calyx, with long, branching arms; the number and arrangement of the component plates are definite and characteristic for each genus.
Most, but not all, of the Crinoids have a long, jointed stem, by which they are attached to the sea-bottom. The earliest Blastoidea, represented by Blastoidocrinus (IV, 12), appear in the Chazy. This genus, a very primitive form which retains notable cystoid-ean characters, is the first member of a group which was to become important in a long subsequent period, the Carboniferous. Asteroids (starfishes) and Ophiuroids (brittle-stars) are found, but cannot be called common, or abundant. The Echinoidea, or sea-urchins, are represented by very primitive forms.
The Trilobites (PI. VII) increase very greatly in the number of genera and species, and most of the Cambrian genera are replaced by new ones. This is the period in which the group of Trilobites attains its highest development, gradually declining afterward and becoming extinct with the close of the Palaeozoic. The most characteristic and widely spread genera of Ordovician Trilobites are: Asaphus, Isotelus (VII, 7), Bu-mastus (VII, 2), Triarthrus (VII, i),Calymmene (VII, 8), Cerau-rus (VII, 6), Trinucleus (VII, 4), Acidaspis (VII, 3), Bronteus (VII, 5), Pterygometopus, etc. These genera differ in aspect from those of the Cambrian in their much larger tail-shields, in their ability to roll themselves up (see VII, 2 a, 8 a), and in their rounder and better-developed, faceted eyes.
Other Crustacea mark advances in the Ordovician. Thus, the Eurypterida, a group which dates from the Algonkian, and was destined to a remarkable development in the Silurian and Devonian, is represented, though not abundantly. Ostracoda and Phyllocarida undergo no marked change. That terrestrial animal life had already begun is demonstrated by the occurrence of an Insect, Protocimex, in Scandinavia. From this we may be assured that terrestial vegetation was already established and that the atmosphere was fitted for the existence of air-breathers.
These shells increase very largely in abundance and variety, the genera with hinged calcareous shells (Articulata) now gaining the upper hand and reducing the horny-shelled kinds to comparative insignificance. The most important genera are: Orthis (V, 11); Platystrophia (V, 12); Dalmanella (V, 14); Plectambonites (V, 15); Rafinesquina (V, 16); Leptcena, Strophomena (V, 17), and RJiynchotrema (V, 19). Spine-bearing shells begin in Zygospira (V, 18).
Plate VI. - Ordovician Mollusca.
Fig. i, Byssonychia radiata Hall. x 1/2, left valve, Trenton. 2, Ambonychiaplanistri-ala Hall, x 1/2, left valve. 3, Opisthoptera fissicosta Meek, x 1/2, right valve, Richmond. 4, Pterinea demissa Conrad, x 1/2, left valve, Trenton. 5, Cyrtodonta huronensis Bill., x 1/2, right valve, Lowville. 6, Cymatonota attenuata Ulrich, x 1/2, right valve, Richmond. 7, Cyclonema humerosum Ulrich, x 1/2, Lorraine. 8, Eotomaria supracin-gulata Bill., x 1/2 9, Trochonema umbilicatum Hall, x 1/2, Trenton. 10. Hormotoma gracilis Hall.x 1/2, Trenton 11, Cyrtolites ornatus Conrad, x 1/2, Lorraine. 12, Proto-warthia cancellata Hall, x 1/2, Black River. 13, Maclurea logani Salter, x 1/2, Trenton. 14, Ophileta compacta Salter, x 1/2, Beekmantown 15, Conularia trentonensis Hall, x 1/2, Trenton. 16, Orthoceras multicameratnm Hall. x 1/8, Lowville. 17, Cyrtoceras juverialis Bill., x 1/2 Trenton. 18, Eurystomites occidentalis Hall, x 1/4. 19, Schrce-deroceras eatoni Whitfield, x 1/2.
This is a group which has yet yielded no representatives from the Cambrian, but appears abundantly in the Ordovician (V, 6-8). The genera differ little from those which live in the modern seas.
One of the most striking differences between the Cambrian and the Ordovician is the great advance made by the Molluscs in the latter period. The Bivalves (Pelecypoda) are larger, more numerous, and more like modern forms (see PI. VI, Figs. 1-6). The Gastropoda likewise increase notably in size and in numbers, especially the spirally coiled shells. Important genera are: Eotomaria (VI, 8); Hormotoma (VI, 10); Pro-towarthia (VI, 12); Trochonema (VI, 9); Maclurea (VI, 13). Neither Bivalves nor Gastropods had anything like the relative importance which they possess in modern times; the latter all had the mouth of the shell forming a complete ring (holosto-mate).
Much the most significant change in the Mollusca, however, is the great expansion of the Cephalopoda, a few of which perhaps appear in the uppermost Cambrian, but in the Ordovician have become one of the predominant elements in the marine life of the times. The Cephalopods, which are the highest group of molluscs, are in modern times represented by two suborders; in one, the squids and cuttlefishes (Dibranchiata), the shell is rudimentary and internal; while in the other (Tetrabranchiata) the shell is external. Such an external shell is divided by transverse septa into chambers, which are connected by means of a tube, the siphuncle, the animal living only in the terminal chamber at the mouth of the shell, the remainder of which is empty. The only existing representative of the Tetrabranchiata is the Pearly Nautilus, but .throughout the Mesozoic and most of the Palaeozoic eras there was an extraordinary variety of these chambered shells. In the Ordovician the Cephalopods were all Nautiloids, most nearly allied to the modern Pearly Nautilus, with chambered shells, divided internally by simple septa.
The commonest shell of this type is Orthoceras, which is a straight and very elongate cone (VI, 16) and sometimes attains a length of 10 feet; the genus persists throughout the Palaeozoic and into the Mesozoic. Endoceras, which likewise has a straight shell, with a curiously complex siphuncle, is confined to the Ordovician. Besides these straight forms we find curved shells like Cyrtoceras (VI, 17), shells like Eurystomites (VI, 18), and Schrcederoceras (VI, 19), which have the young shell coiled and the portion formed in old age straight, resembling an Orthoceras with its smaller end rolled up into a coil. Others again, like Trocholites, have the shell coiled in a close, flat spiral.
Plate VII. - Ordovician Trilobites.
Fig. i, 1a, Triarthrus becki Green, x 3/2, Utica. Restoration by Beecher of dorsal and ventral sides. 2, Bumastus trentonevsis Emmons, x 1/2, Trenton. 2a, The same, from the side, rolled up. 3. Acidaspis crosotus Locke, x 4, Richmond. 4, Tri'nucleits concen-tricus Eaton, x 1, Trenton. 5, Bronteus lunatus Bill., x 1, Trenton. 6, Ceraurus pleu-rexanthmus Green, x 1, Trenton. 7, Isotelus maximus Locke, x 1, Trenton. 8, Calyni' toene callicephala Green, x 1, Richmond. 8a, The same, rolled up, from the side.
A peculiar shell, Conularia (VI, 15), which has a four-sided, pyramidal shape, with four triangular pieces to close the mouth, is a genus referred to the Pteropoda.
The curious, mail-clad Ostracoderms, primitive vertebrates which somewhat resemble the fishes in appearance, have been found in the Ordovician sandstones of Colorado and Wyoming. As these remains are imperfect, description of the Ostracoderms will be deferred till a later chapter.