Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/369

Rh JURAss1c.] changes which had taken place in the long interval. The brachiopods are chiefly species of I.’li_i/nchonclla and Tcrcbratula—gei1cra which, though occurring in 1’al:cozoic rocks, play there a comparatively subordinate part. They include the last Spz'r1_'fc'r1_3 and Lcplwnw. Uf the laiiicllibranehs, a few of the most characteristic genera are (/r3/plucrc, Lima, Pcclcn, Zllonot-is, Cariliiria, Ilippopodim/2., ilfyacilcs, and Pholiulonig/a. Gastcropods, though usually rare in such muddy strata as the greater part of the Lias, occasionally occur, but most frequently in the calcareous zones. Altogether 89 species are at pi-e.~:cnt known, the chief genera being I’lcurotomarz'a, Turbo, /;'lU'Z//‘Ills, C'r.'ri'I/u'um, and C'hcmnz't:z'a. The ceplialopods, however, are the most abundant and characteristic shells of the Lias; the family of the ammonites numbers in Yorkshire at least 113 species. As already stated, some species are so distinctive of special horizons that the whole of the Lias has been subdivided into zones, each denoted by the name of its cliaracte1'istic aiiiiiioiiitc. In ascending order, these zones are named as follows :—In the Lower Lias,—1st, A/n.mmu'lcs ;2lan.orI/1'3 ; 2d, A. an._r/1Llati(s; 3d, A. Bucklandi; 4th, A. tuberculatus; 5th, A. obtusus; 6th, A. 0.L’y)l0ll(S; 7th, A. z'a7‘[co.s- (talus. In the Middle Lias,—lst, A. Jwmcsoni and (u'7)l(Llll8 ; ‘Zd, A. erI}n'z'cor/Lus; 3d, A. nwrgan'tatics; 4th, A. spinatus; 5th, A. annu- lufux-. ln the Upper Li-.1s,——lst, A. scrpcntinus; 2:1, A. cominimis; 3d, .4. ju/'c:z.s-1'3. The genus .T¢r..utilus is also present. The di- l')l':lll(‘lllilt“ cephalopods are represented by upwards of 40 species of the genus 1?:-lrnznifcs. Numerous species of ﬁshes have been obtained from the Lias. Some of these are placoids, known only by tlicirtecth (.-fL'I'"lll.S‘, Uzratodus), others only by their spines (Xe:/zacaaztlzus), and soine by both teeth and spines (I13/bodus). The ganoids are fri-qiieiitly found entire, the genera Drqicclizts, J’I1.oli¢lop/torus, 4‘EC/(IIl0(fl€.9, L(’1)l:(l0tl($‘, Pachycorui-us, and Lcptolc/ais being among the most frequent. But undoubtedly the most important palze- ontologi--al feature of the Lias is the number and extraordinary interest of its reptilian remains. These include the extinct order of cnaliosaiiis or sea-lizards, uniting characters which are not found t0_'_(‘i'tllCl' in any living forms. Among these are the genus IeIi(/2_i/o- sruzrus, a creature with a fish-like body, two pairs of strong swim- ming p-iddles, and probably a vertical tail-fin. The head, joined to the body without any distinct neck, was furnished with two large eyes, having a ring of bony plates round the eyeball, and with teeth that had no distinct sockets. Some of the skeletons of this creature exceed '2-1 feet in length. Contcinporancous with it was the Plnsiusaurus, distiugiiished by its long neck, the larger size of its pzulclles, the smaller size of head, and the insertion of its teeth in special sockets, as in the lngher sauriaiis. These creatures seem to have haunted the shallow seas of the Liassie time. There were al.-.o_linge winged ba_t-l_ikc reptiles (Dimorp/zodon), with large heads, liaving teeth in distinct sockets, eyes like the Iehlhg/osaur-us, and one finger of e_ar:h fore foot prolonged to a great length, for the purpwse of supporting a membrane for ﬂight. The bones, like those 01 birds, were hollow and air-ﬁlled. Gigantic deinosanrs were like- wise among the inhabitants of Britain during Liassie time. These were true reptiles, yet with peculiarities of structure, particularly in the hinder part of the skeleton, linking them with birds like the ostrich. To this order belongs the Megalosaurus from the Lias of Lyme liegis. Lastly, the true crocodiles had representatives in the Li-assie waters and shores, in the genera Tclcosaur-its and Simeo- S((.l(I'l(S. The LOWER or BATH OoL1TEs lie conformably upon the top of the Lias, with which they are connected by a general similarity of organic remains. Out of the 313 known species in the Upper Lias, about 39 or 40 pass up into the overlying formation. The lowest of the three subdivisions of the English Oolites consists in the south- west and centre of England of shelly marine limestoiies, with clays and sandstones; but, as these strata are traced iiortliw.irds into N orthaiiipton, Rutland, and Lincolnshire, they pass into a series of strata indicative of deposit in the estuary of some river descending from the north, for, instead of the abundant cephalopods of the truly marine and typical series, we meet with fresli—water genera such as ('_z/rcna and Unzo, with marine forms such as O.s(rea and illodzola, thni seams of lignite, and remains of terrestrial plants. These indications of the proximity of land become still more marked in Yorkshire, where the strata (800 feet thick) consist chieﬂy of sandstones, shales with seams of ironstone and coal, and occasional horizons containing inarine shells. It is deserving of notice that the Coriibrasli, which forms the top of the Lower Oolite in the typical Gloiiccstcrsliire district, occurs likewise in the same position ' 7' ' a I in lorksliire. Though rarely more than 8 feet thick, it runs across the country from Devonshire to Yorkshire. GEOLOGY 355 Thus a distinctly deﬁned series of beds of an estuarine character, exactly representative of the marine formations of the south-west-, shows us that at the close of the Lower Oolitic period the estuary of the northern tract was sub- merged, and a continuous sea-ﬂoor stretched across the whole of the south-east of England. Thanks to the deposits of the Yorkshire estuarine series our knowledge of the Oolitic is much more ample than of the Liassic Ilora. With the exception of a few littoral fucoids all the plants are of terrestrial forms. They comprise about 60 species of ferns, among which the genera Peeoplcris, Sphc7zoptdrz's, Plzlcboptcv--is, and Tiemuptcris are characteristic. N ext in abundance come the eycads, of which more than 20 species are known, belonging to the genera Otoza-mites, Zamitcs, Ptcrophyllum, and Cycaditcs. (‘on- ifcroiis remains are not infrequent in the form of stems or frag- ments of wood, as well as in occasional twigs with attached leaves; the genera A raucarites, Brachyp/zyllmn, Thug/tcs, Pczzcc, IValc/via, C'r_uptomcritcs, and Tmritcs have been recognized. The Inferior Oolite presents a tolerably copious suite of invertebrate remains, which resemble generically those of the Lias. The predominance of 1.’./2.yiwho7wlla and Tercbratula over the rest of the brachiopods becomes still more marked. Gryphara, Lz'1na, Pcetcn, Uarcliiun, Jlyacitcs, illytilus, Pholadomya, Tr-igomia are frequent shells, while ammonites and belemnites also occur, though much more sparingly than in the Lias below, and in some of the later subdivisions of thc Oolitic series. The Fuller's Earth, though well-marked in the Bath district, where it is about 200 feet thick, dies out in Oxfordshire, and contains only a few distinctive fossils, most of its forms being also found in the Inferior Oolite. The Great or Bath Oolite consists, in Gloucestershire and Oxfordshire, of three groups..of strata. At the base comes a series of thiii—bedded limestones with sands, known as the Stonesﬁcld Slate; in the centre lies a mass of shelly and yellow or cream-coloured often Oolitic liinestoiies, with partings of marl or clay—the Great Oolite ; while at the top lies a set of clays and shelly limestones, including the Bradford Clay, Forest Marble, and Coriibrasli. The Stones-- ﬁeld Slate, the lowest of these three zones, is alocal but ex- ceedingly important subdivision, which has furnished a large number of reptilian and some mammalian remains. lt must have been deposited in shallow water close to thickly wooded shores. About a dozen species of fc1'ns have been found in the Stoncsﬁcld Slate, the genera Pecoptcris, Sphcnoptcris, and Taenioptcris being still the prevalent forms. The eycads are qlliieflyd species of Pafl_ceozamz'a, the conifers of Thuytcs. Vith these rifte ragnicnts o a terres- trial vcgctation therc occur remains of beetl 5, dragon-ﬂies, and other insects which had been blown or washe off the land. The waters were tenanted by a few bracliiopods (lfhynclzonclla and Tc:-cbratula), by laincllibranclis (G'er'cilli'a, Lima, Ostrca, Pecten, A.st(u'lc, illodiola-, Trigo/Lia, &c.), by gasteropqdis) §Nat[ca, Nerlitlgc, Palclla Trochus Sze. , b a few ammonites am e emnites, am y placoid, and ganbid tishe).:i, of which about 50 species are known. The reptiles comprise representatives of turtles, with peculiar species of Ielzt/zyosaurus and Plesiosaurus. The genus Telcosam-us, which occurs in the Yorkshire Lias, is among the organisms of the Stones- ﬁeld Slate. It was a true carnivorous crocodile, measuring_ about 18 feet in Icngtli, aml is judged by Phillips to have been in the habit of venturing more freely to sea than the gavial of the Gauges and the crocodile of the Nile. The huge deinosaur lllcgalosawrus frequented the shores of the Stonesﬁeld lagoons, walking probably on its massive hind legs, and feeding on the mollusks, fishes, and perhaps the small mammals pf thei district. 1It1l%eStln§a:€(l to l1ri_1‘lVC had a leneth of 25 feet, am to iavc weie iei .. or ons. ie ﬂying reptziles were likewise represented by the R/icmzphorhyneh-us,— a harpy-like creature which was able to ﬂy, to shuﬂle on land, or perch on rocks, perhaps even to divp iii seiéreh ot;ii’c1i1}i§%yt. But the most im ortant or anie remains o tic etones e a e are un- doubtedlg its inamgnalia, of which three genera A inpIn't/icrium, P/Lascolotheriunz, and Stcrcognat/ius have been determined. Only portions of lower jaws lpive yctdbcpn fiopnd, poipltiplg dioubtlcss to the fact that, as t 1e anima swerc ri tei rom an, ie ower_]aws, un- protected by outer skin, were separated in decomposition from the rest of the body. These intenesting relics were the first traces of mammalian life found in strata of such high antiquity. They are regarded as having belonged to small niarsupial animals, to which living analogues exist in Australia. In the Great Oolite the re- mains of a gigantic saurian Cctcosaurils Iiave been found. Accord- ing to Phillips it was probably, when standing, _not less.than_10 feet in height and 50 feet in length, a marsh-loving or river-side animal, living on the ferns, eycads, and conifers among which If dwelt.