Geology . ods,(4) the rejuvenation of the corals and crinoids, (5) the marked develop-ment of the sea-urchins, (6) the introduction of crabs and moderntvpes of crustaceans, (7) the prevalence of foraminifera, radiolariansand sponges, and (8) the change in the aspect of the fishes, while(9) all were dominated by the great sea-serpents evolved from theland-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 amonginvertebrates, and were represented by many beautiful forms. Theydeployed on ascending
Geology . ods,(4) the rejuvenation of the corals and crinoids, (5) the marked develop-ment of the sea-urchins, (6) the introduction of crabs and moderntvpes of crustaceans, (7) the prevalence of foraminifera, radiolariansand sponges, and (8) the change in the aspect of the fishes, while(9) all were dominated by the great sea-serpents evolved from theland-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 amonginvertebrates, and were represented by many beautiful forms. Theydeployed on ascending lines in some cases, and retrogressive lines inothers. There were cases of erratic and senile development, reflected THE JURASSIC PERIOD. 81 by uncoiling, spiral coiling, and other departures from the normallines of the order, presaging an episode of sporting and retrogressionin the next period, to be followed by extinction; but, despite theseadverse foreshadowings and some notable reduction in diversity,. Fig. 357.—A Group of Jurassic Ammonites: a-b, Coroniceras bisulcatum (Brug.),a lateral and ventral view of one of the Arietidce; c, Deroceras subarmatum (Young);d, Perisphinctes tiziani (Oppel); e, Reineckia brancoi Steinm. the ammonites were yet in the climacteric stage of their luxurianceand beauty. They had well-nigh reached the limits of attainmentin such features as close coiling, complexity of sutures, ornamenta-tion and some other characteristics. The continued expansion ofthe sea gave them still a widening field over which they spread them- 82 GEOLOGY. selves in successive4 Fig. 358.—The internalshell of a belemnite,restored; the lower,solid, conical portion,the part most fre-quently preserved, isthe rostrum or guard;the middle portion isthe phragmocone,which is a diminutivechambered shell withsepta, siphuncle, andprotoconch as in theolder tetrabranch or-der; the upper part isthe prostracum, whichcorresponds to the pen of the livingcuttle-fishes. of both continen
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