DEVELOPMENT OF FAMILIES, ETC., IN TIME. 



243 



coverv or the caprice of opinion. Tlie discovery of new types 

 is not likely to be frequent ; the imposition of new names, in 

 place of the old, will not increase the number of Palaeozoic 

 genera ; and the establishment of fresh and arbitraiy distinctions 

 will atfect all the groups in due proportion. 



If the number of groups called " Systems" were reduced to 

 seven (viz., three Pakeozoic, three Secondary, and one Tertiary, 

 as shown in the following table), then the average duration of a 

 genus of shells would be equal to a System of Formations. 



The duration of the smallest well-defined Families of shells is 

 about equal to one of the three great geological divisions, or 

 ages. — Woodward. 



DEYELOPMENT OF FAMILIES, GENERA, AND 

 SPECIES, IN TIME. 



Geological 

 Systems. 



E-t D 



O 



Oh 



(Cambrian 49 12 



(Silurian 53 13 



Devonian 77 



( Carboniferous 

 ( Permian .... 



14 

 79 11 

 66 (S 



{ 4 Trias 81, 9 



5 ^ 



(L.Jurassic 107 12 



C pq 



11 15 

 11! 16 

 20 23 



261 19 



24i 16 



I 



25 16 

 35 12 



g ^ " \U. Jurassic . . . .108 13 36 9 

 8 I r. ( L. Cretaceous . .123 20 4l! 9 

 ^ [ lIJ. Cretaceous . .148' 16 59 14 



11 

 13 

 20 

 23 



20 



31 

 48 

 50 

 53 

 59 





(Eocene 172; 4- 85 11 72 



7 \ Miocene 178! 3 97! 11 I 76| 



(Pliocene 192^ 1 100^12 1 79 



Recent 400 21 25l| 13 115 



I 1 I 



Recent and Fossil. 520 56 280 34 !l50 



Total 

 Number of 



Species 

 (I'Orbigny). 



imilics. 





fe 



362 



18 





317 



20 



^32 



1035 



24 



835 



30 





74 



30 





713 



351 





1502 



42 





1266 



49 



^57 



784 



52 





2147 



56 J 





2636 



60] 





2242 



62 r^ 



437 



16,000 



78] 



- 



30,000 



85 



A few words as to certain relations of fossil faunas with those 

 now existing, will close this portion of our work. 



Phillips has said that the Jurassic period has its parallel in 

 the existing fauna of Australia. The Jurassic mammals are, in 

 fact, Didelphians ; and among Jurassic marine moUusks, Tri- 

 gonia still exists in Australian seas. 



