80 GEOLOGY. 



The Jurassic L t fe. 



As the Jurassic seems to have been mainly a period of sea extension, 

 correlated with a base-leveling of the land, the marine life again assumes 

 a place of leading importance. At the same time the land life, though 

 suffering somewhat by the limitation of its territory during the stages 

 of sea transgression, was favored by the subdued attitude of the land 

 and the genial climate. The frequent shif tings of land- and sea-areas, 

 without involving great topographic relief or severe climatic states, 

 conduced to changes in the forms of life which were on the whole pro- 

 gressive and expansional, though necessarily retrogressive in particular 

 phases. 



The Marine Life. 



It will be recalled that a markedly expansional stage of epiconti- 

 nental sea life had set in toward the close of the Trias. This held 

 on into the Jurassic, fluctuating with the sea expansions and retro- 

 gressions, but in general progressing until it reached a climax in the 

 latter part of the period, when the sea attained the limit of its remark- 

 able transgression upon the land. Later there was a measurable 

 decline closing the period. As already indicated, this faunal progress 

 is far less well revealed in North America than in Europe and Asia, 

 and a general sketch drawn chiefly from the Old World may well pre- 

 cede a special statement of the more meager American development. 



The great features of the marine life lay in (1) the continued domi- 

 nance of the ammonites among the invertebrates, (2) the rise of the 

 belemnites, (3) the abundance and modernization of the pelecypods, 

 (4) the rejuvenation of the corals and crinoids, (5) the marked develop- 

 ment of the sea-urchins, (6) the introduction of crabs and modern 

 types of crustaceans, (7) the prevalence of foraminifera, radiolarians 

 and sponges, and (8) the change in the aspect of the fishes, while 

 (9) all were dominated by the great sea-serpents evolved from the 

 land-reptiles of the Trias. 



(1) The ammonites which, in certain respects, reached their cli- 

 max in the later stages of the Trias, were still the master type among 

 invertebrates, and were represented by many beautiful forms. They 

 deployed on ascending lines in some cases, and retrogressive lines in 

 others. There were cases of erratic and senile development, reflected 



