FIRST JURASSIC PERIOD. 



time of the family of Calmars, which include the Belemnites 

 and various new bivalve Molluscs. 



339. In the seas existed also madreporic reefs or islands similar 

 to those which exist at present in intertropical regions (fig. 202). 

 Lakes were numerous and were inhabited by tribes of fresh 

 water Testacea, whose remains are now found in the Portland beds 

 and elsewhere. 



340. Insects of the orders Diptera (flies, gnats), Hymenoptera 

 (bees, ants), and Hemiptera (bugs, water scorpions, plant lice), 

 Crustaceous Isopods (sow bugs), Tectibranche-Molluscs, Cirride- 

 Brachiopods, free Crinoids, severally appeared in this age for the 

 first time upon the earth. 



341. The vegetable kingdom presented none of the character- 

 istics of the preceding. The colossal ferns and lycopodiacere 

 had disappeared, and had been replaced by new species of the 

 same families. Conifers were abundant in the Lias period, with 

 new species of CicadeEe. The fruit of a species of palm is found 

 in the same deposit, as well as carbonaceous strata, altogether 

 different from those of the carboniferous period. The difference 

 of the quantity as well as the quality of these vegetable deposits 

 indicate in a striking manner the difference of the extent of the 

 continents of the two periods. 



This general account of the Jurassic age will render necessary 

 only a very brief notice of its successive periods. 



FIRST JURASSIC PERIOD. 



342. The geological convulsion which closed the Triassic age 

 having destroyed the animal and vegetable kingdoms, and the 

 tumult having been followed by a period of tranquillity, the seas, 

 subsiding within their limits, deposited the strata which form the 

 first Jurassic stage. These deposits, called by D'Orbigny the 

 Sinemurian stage, from the circumstance of their great develop- 

 ment in the neighbourhood of the town of Semur (Sinemurium), 

 correspond with the lower lias of British geologists, the gryphite 

 limestone of Dufresnoy, Elie de Beaumont, and Roemer, and 

 the quadersandstein of other German geologists. 



The parts of Europe where this stage appears at the surface 

 round the great Jurassic basins are shown upon the map (fig. 153), 

 by the shading marked 7, being the external boundary of the 

 series of concentric lines surrounding these basins. In England 

 this stage is seen in a continuous zone, extending N.N.E. from 

 Lyme Regis, through the counties of Dorset, Somerset, Gloucester, 

 "Worcester, Warwick, Leicester, Nottingham, and Lincoln. It 



77 



