5 6 4 



HISTORICAL GEOLOGY 



have been considered by some students as reptilian and by others 

 as mammalian. If they are reptilian, they are theromorphs; if 

 mammalian, either monotremes (egg layers that suckle their young, 



like Platypus), or marsupials (mam- 

 mals that produce their young in an 

 immature condition, like the opos- 

 sum). It is suggestive, however, to 

 Fig. 524. — Jaw of either a primi- note that creatures that are either 

 tive mammal or a theromorph reptile rept il es w i tn strong mammalian char- 

 (Inassic), twice the natural size. 1 • 1 11 1 1 



(After Emmons.) acters or mammals with well-marked 



reptilian characters existed before 

 true mammals made their appearance. With one exception, the few 

 mammalian remains found in the Mesozoic are of small size, indicat- 

 ing animals not larger than rats. Those of the Jurassic and Creta- 

 ceous appear to be insectivores and to be either monotremes (egg- 

 laying mammals) or marsupials, but none clearly belong to the 

 highest type of mammals (Eutheria, common mammals of to-day) 

 nor are they closely related to other forms. 



PALEOZOI C 



MESOZOIC 



1 CENOZOIC 



Cambrian 



Ordovician 



Silurian Devonian 



Mississippian 



Pennsylvariian 



Permian 



Triassic 



Jurassic 



Cretaceous 



| Tertiary 



Quaternary 





















.vV 



















Jr/ 







$P^ 























JP 





















V 



/ 



L^-""""" 1 





^-■^ 



#p 



^ 











/ tv 



e«o*°^ 



^^~- 



-"foOTHEO 



3IRDS„' J 



. 









ANCESTRAl 

 FISH 



^x^ 



-„_^ 



/ 









^r 



PTERODAC 



TYLS 







PRIMITIVE 



__?iPTUJ 



_S 













^*o^ 



5 





__JELE( 



STS 









X-EXT 



NCT 





















-►-LIVI 



MG AT P 



»ESENT 

















~~-— -^ 



Tab 



le shoi 



ving ti 



ne of a 



apearar 



ce anc 



proba 



>le 











on 



ain of 



the var 



ious c 



lasses 



of verf < 



brare; 













Fig. 525. — Table showing the probable relationships of vertebrates, with their 

 geological distribution. 



