CENOZOIC ERA: AGE OF MAMMALS 



627 



out the Tertiary, being, as now, the most numerous of the larger 

 invertebrates, During the period they became more and more like 

 the living forms, until in the Pliocene they were nearly identical 

 with those of to-day. Certain species are characteristic of the various 

 formations of the epoch and are consequently of great stratigraphic 

 value. Some Miocene 



pelecypods attained a .^f "^w 



large size; oysters 13 

 inches long, 8 inches 

 wide, and 6 inches thick, 

 as well as pectens (Fig. 

 558 /) 9 inches in diam- 

 eter are known. Large 

 size was not, however, 

 characteristic of the pe- 

 lecypods of the period. 

 Larger pelecypods are 

 living to-day than in any 

 previous period. 



The cephalopods were 

 represented by the nau- 

 tilus and squid, the 

 former having a wider 

 distribution than now. 



Insects. — At the present about 400,000 living species of insects 

 have been described, but less than 8000 fossil species are known. The 

 small number of fossil insects as compared with those of the present 

 does not indicate that this class is more numerous to-day than at 

 certain times in the Tertiary, but rather that few of the Tertiary 

 species have been preserved. It has even been suggested that owing 

 to the warmer climate and more luxuriant vegetation, and judging 

 from the proportion of species, the total insect fauna of the Miocene 

 of Europe may have been greater, in some respects, than it is now 

 in any part of that continent. The greater number of Tertiary 

 insects have been preserved in amber, in which they were entrapped 

 when the gum of the trees on which they were crawling was first 

 exuded and was soft and sticky. About 2000 species have been thus 

 preserved, some of the specimens of which are in an almost perfect 

 state of preservation, all of the external characters being as well 

 shown as in life; others are preserved in peat; and still ethers in 



Fig. 557. — Tertiary echinoid : Scutella aberti 

 (Miocene). (Maryland Geol. Surv.) 



