424 



THE GENERAL PRINCIPLES OF ZOOLOGY 



have already noted in Chapter II, it is divided into three periods 

 or formations, the Triassic or oldest, the Jurassic, and the Cre- 

 taceous or most recent. 



While many genera and in some cases entire orders of inver- 

 tebrates become extinct in the Palaeozoic, many persist into the 

 Mesozoic, and new groups appear. The Brachiopoda are much 

 less numerous than in the preceding era. The Crustacea 

 appeared in the Cambrian, lost the Trilobitas and some of their 



allies (Fig. 402) before the 

 end of the Palaeozoic, were 

 increased by the addition of 

 the Xiphosura, or king crabs 

 (Fig. 403), in the Triassic. 

 and by the true decapod 

 crabs in the Cretaceous. 

 Orders of insects which exist 

 to-day first made their ap- 

 pearance in the Triassic, 

 where we find the Orthop- 

 tera, Neuroptera, and Cole- 

 optera ; the Hemiptera, 

 Lepidoptera, Diptera, and 

 Hymenoptera appear in the 

 Jurassic. Many new genera 

 of the Mollusca are found 

 in the Mesozoic; the Nauti- 

 loids whose great develop- 

 ment in the Palaeozoic we 

 have noted, have nearly dis- 

 appeared in the Triassic even ; in them the* septa which divide 

 the shell into chambers are smooth and curved, and the lines 

 or sutures where they meet the wall of the shell are simple 

 curved lines. Clearly related to the Nautiloids, is another group 

 called the Ammonites, having a similar shell, but with folded 

 septa, and sutures which are irregular, lobed, or even foliaceous. 

 Some of the Ammonites appear in the Palaeozoic, but the 

 majority are confined to the Mesozoic, where they attain a high 

 degree of development, becoming extinct before the next era. 

 The shape of the shell varies much in the fossil tetrabranchiate 



FlG. 403. Limulus polyphenols, the horseshoe 

 crab, adult female; actual length, forty-six centi- 

 meters. (Photographed from a dried specimen 

 by the author. ) 



