PREHISTORIC ANIMALS 



423 



orders should have come into existence, and that some should 

 even have attained their maximum development in number of 

 species and differentiation of structure, and have died out so 

 that they do not appear in succeeding eras. This is true of two 

 classes of the Echinodermata, the Cystoidea and the Blastoidea 

 (Figs. 399 and 400), while the Crinoidea were much more abun- 

 dant then than at any ... , 

 later period ; their re- 

 mains form great beds 

 of limestone in the 

 Silurian and Carbon- 

 iferous periods. 



Another group 

 which became extinct 

 in the Palaeozoic is the 

 Trilobitae (Fig. 401), 

 already referred to in 

 Chapter VII in con- 

 nection with the Crus- 

 tacea, and the same is 

 true of the various 

 insects which lived at 

 that time ; they are 

 known collectively as 

 the Palaeodictyoptera 

 (Gr. iraXaios, ancient, 

 &tKTvov,net, and7TTe/3oV, 

 wing). Of the verte- 

 brates the Ostraco- 

 dermi are not found 

 above the Permian. 

 But in all the Palaeo- 

 zoic rocks, requiring so long a period for their formation, there 

 is no trace of the remains of the warm-blooded animals, the 

 birds and mammals. These first appear in the next era, the 

 Mesozoic, which comprises for the most part animals inter- 

 mediate in structure between the Palaeozoic and the Cenozoic. 

 The estimated age of the Mesozoic era, based on the same data 

 as the age of the Palaeozoic, is 7,200,000 years, and, as we 



FIG. 402. Burypterus fisckeri, from the Silurian. 

 Parker and Haswell's Text-book.) 



( From 



