Corals were abundant, and in later portions of the period covered
large areas in Europe; the modern type of coral became dominant;
besides reef-building forms such as Thamnastrea, Isastrea, Thecosmilia,
there were numerous single forms like Montivaltia. Crinoids
existed in great numbers in some of the shallow seas; compared with
Palaeozoic forms there is a marked reduction in the size of the
calyx with a great extension in the number of arms and pinnules;
Pentacrinus, Eugeniacrinus, Apiocrinus are all well known; Antedon
was a stalkless genus. Echinoids (urchins) were gradually developing
the so-called “irregular” type, Echinobrissus, Holectypus,
Collyrites, Clypeus, but the “regular” forms prevailed, Cidaris,
Hemicidaris, Acrosalenia. Sponges were important rock-builders
in Upper Jurassic times (Spongiten Kalk); they include lithistids
such as Cnemediastrum, Hyalotragus, Peronidella; hexactinellids,
Tremadictyon, Craticularia; and horny sponges have been found in
the Lias and Middle Jurassic.
Polyzoa are found abundantly in some of the beds, Stomatopora, Berenicia, &c. Brachiopods were represented principally by terebratulids (Terebratula, Waldheimia, Megerlea), and by rhynchonellids; Thecae, Lingula and Crania were also present. The Palaeozoic spirifirids and athyrids still lingered into the Lias. More important than the brachiopods were the pelecypods; Ostrea, Exogyra, Gryphaea were very abundant (Gryphite limestone, Gryphite grit); the genus Trigonia, now restricted to Australian waters, was present in great variety; Aucella, Lima, Pecten, Pseudomonotis Gervillia, Astarte, Diceras, Isocardia, Pleuromya may be mentioned out of many others. Amongst the gasteropods the Pleurotomariidae and Turbinidae reached their maximum development; the Palaeozoic Conularia lived to see the beginning of this period (Pleurotomaria, Nerinea, Pteroceras, Cerithium, Turritella).
Cephalopods flourished everywhere; first in importance were the ammonites; the Triassic genera Phylloceras and Lytoceras were still found in the Jurassic waters, but all the other numerous genera were new, and their shells are found with every variation of size and ornamentation. Some are characteristic of the older Jurassic rocks, Arietites, Aegoceras, Amaltheus, Harpoceras, Oxynoticeras, Stepheoceras, and the two genera mentioned above; in the middle stages are found Cosmoceras, Perisphinctes, Cardioceras, Kepplerites Aspidoceras; in the upper stages Olcostephanus, Perisphinctes, Reineckia, Oppelia. So regularly do certain forms characterize definite horizons in the rocks that some thirty zones have been distinguished in Europe, and many of them can be traced even as far as India. Another cephalopod group, the belemnites, that had been dimly outlined in the preceding Trias, now advanced rapidly in numbers and in variety of form, and they, like the ammonites, have proved of great value as zone-indicators. The Sepioids or cuttlefish made their first appearance in this period (Beloteuthis, Geoteuthis,) and their ink-bags can still be traced in examples from the Lias and lithographic limestone. Nautiloids existed but they were somewhat rare.
A great change had come over the crustaceans; in place of the Palaeozoic trilobites we find long-tailed lobster-like forms, Penaeus, Eryon, Magila, and the broad crab-like type first appeared in Prosopon. Isopods were represented by Archaeoniscus and others. Insects have left fairly abundant remains in the Lias of England, Schambelen (Switzerland) and Dobbertin (Mecklenburg), and also in the English Purbeck. Neuropterous forms predominate, but hemiptera occur from the Lias upwards; the earliest known flies (Diptera) and ants (Hymenoptera) appeared; orthoptera, cockroaches, crickets, beetles, &c., are found in the Lias, Stonesfield slate and Purbeck beds.
Fishes were approaching the modern forms during this period, heterocercal ganoids becoming scarce (the Coelacanthidae reached their maximum development), while the homocercal forms were abundant (Gyrodus, Microdon, Lepidosteus, Lepidotus, Dapedius). The Chimaeridae, sea-cats, made their appearance (Squaloraja). The ancestors of the modern sturgeons, garpikes and selachians, Hybodus, Acrodus were numerous. Bony-fish were represented by the small Leptolepis.
So important a place was occupied by reptiles during this period that it has been well described as the “age of reptiles.” In the seas the fish-shaped Ichthyosaurs and long-necked Plesiosaurs dwelt in great numbers and reached their maximum development; the latter ranged in size from 6 to 40 ft. in length. The Pterosaurs, with bat-like wings and pneumatic bones and keeled breast-bone, flew over the land; Pterodactyl with short tail and Rhamphorhyncus with long tail are the best known. Curiously modified crocodilians appeared late in the period (Mystriosaurus, Geosaurus, Steneosaurus, Teleosaurus). But even more striking than any of the above were the Dinosaurs; these ranged in size from a creature no larger than a rabbit up to the gigantic Atlantosaurus, 100 ft. long, in the Jurassic of Wyoming. Both herbivorous and carnivorous forms were present; Brontosaurus, Megalosaurus, Stegosaurus, Cetiosaurus, Diplodocus, Ceratosaurus and Campsognathus are a few of the genera. By comparison with the Dinosaurs the mammals took a very subordinate position in Jurassic times; only a few jaws have been found, belonging to quite small creatures; they appear to have been marsupials and were probably insectivorous (Plagiaulax Bolodon, Triconodon, Phascolotherium, Stylacodon). Of great interest are the remains of the earliest known bird (Archaeopteryx) from the Solenhofen slates of Bavaria. Although this was a great advance beyond the Pterodactyls in avian characters, yet many reptilian features were retained.
Comparatively little change took place in the vegetation in the time that elapsed between the close of the Triassic and the middle of the Jurassic periods. Cycads, Zamites, Podozamites, &c., appeared to reach their maximum; Equisetums were still found growing to a great size and Ginkgos occupied a prominent place; ferns were common; so too were pines, yews, cypresses and other conifers, which while they outwardly resembled their modern representatives, were quite distinct in species. No flowering plants had yet appeared, although a primitive form of angiosperm has been reported from the Upper Jurassic of Portugal.
The economic products of the Jurassic system are of considerable importance; the valuable coals have already been noticed; the well-known iron ores of the Cleveland district in Yorkshire and those of the Northampton sands occur respectively in the Lias and Inferior Oolites. Oil shales are found in Germany, and several of the Jurassic formations in England contain some petroleum. Building stones of great value are obtained from the Great Oolite, the Portlandian and the Inferior Oolite; large quantities of hydraulic cement and lime have been made from the Lias. The celebrated lithographic stone of Solenhofen in Bavaria belongs to the upper portion of this system.
See D’Orbigny, Paléontologie française, Terrain Jurassique (1840, 1846); L. von Buch, “Über den Jura in Deutschland” (Abhand. d. Berlin Akad., 1839); F. A. Quenstedt, Flötzgebirge Württembergs (1843) and other papers, also Der Jura (1883–1888); A. Oppel, Die Juraformation Englands, Frankreichs und s.w. Deutschlands (1856–1858). For a good general account of the formations with many references to original papers, see A. de Lapparent, Traité de géologie, vol. ii. 5th ed. (1906). The standard work for Great Britain is the series of Memoirs of the Geological Survey entitled The Jurassic Rocks of Britain, i and ii. “Yorkshire” (1892); iii. “The Lias of England and Wales” (1893); iv. “The Lower Oolite Rocks of England (Yorkshire excepted)” (1894); v. “The Middle and Upper Oolitic Rocks of England (Yorkshire excepted)” (1895). The map is after that of M. Neumayr, “Die geographische Verbreitung der Juraformation,” Denkschr. d. k. Akad. d. Wiss., Wien, Math. u. Naturwiss., cl. L., Abth. i, Karte 1. (1885). (J. A. H.)
JURAT (through Fr. from med. Lat. juratus, one sworn, Lat.
jurare, to swear), a name given to the sworn holders of certain
offices. Under the ancien régime in France, in several towns, of
the south-west, such as Rochelle and Bordeaux, the jurats were
members of the municipal body. The title was also borne by
officials, corresponding to aldermen, in the Cinque Ports, but is
now chiefly used as a title of office in the Channel Islands. There
are two bodies, consisting each of twelve jurats, for Jersey and
the bailiwick of Guernsey respectively. They are elected for
life, in Jersey by the ratepayers, in Guernsey by the elective
states. They form, with the bailiff as presiding judge, the royal
court of justice, and are a constituent part of the legislative
bodies. In English law, the word jurat (juratum) is applied to
that part of an affidavit which contains the names of the parties
swearing the affidavit and the person before whom it was sworn,
the date, place and other necessary particulars.
JURIEN DE LA GRAVIÈRE, JEAN BAPTISTE EDMOND
(1812–1892), French admiral, son of Admiral Jurien, who served
through the Revolutionary and Napoleonic wars and was a peer
of France under Louis Philippe, was born on the 19th of November
1812. He entered the navy in 1828, was made a commander
in 1841, and captain in 1850. During the Russian War he commanded
a ship in the Black Sea. He was promoted to be rear-admiral
on the 1st of December 1855, and appointed to the
command of a squadron in the Adriatic in 1859, when he absolutely
sealed the Austrian ports with a close blockade. In
October 1861 he was appointed to command the squadron in
the Gulf of Mexico, and two months later the expedition against
Mexico. On the 15th of January 1862 he was promoted to be
vice-admiral. During the Franco-German War of 1870 he had
command of the French Mediterranean fleet, and in 1871 he was
appointed “director of charts.” As having commanded in chief
before the enemy, the age-limit was waived in his favour, and he
was continued on the active list. Jurien died on the 4th of
March 1892. He was a voluminous author of works on naval
history and biography, most of which first appeared in the Revue
des deux mondes. Among the most noteworthy of these are
Guerres maritimes sous la république et l’empire, which was translated
by Lord Dunsany under the title of Sketches of the Last Naval
War (1848); Souvenirs d’un amiral (1860), that is, of his father,
