262 GEOLOGICAL BIOLOGY. 



Genera making their Initial Appearance in each Era. — The 



generic expansion kept up with greater force, as the number 

 of genera making their initial appearance testifies. For the 

 ten successive eras the initiations of new genera recorded 

 up to the present time are as follows, viz. : 5, 25, 37, 34, 34, 

 24, 33) II) 2, 4 for Quaternary and 11 for Recent. The 

 greater number of recent genera not known in fossil state 

 may be discounted by the vastly greater knowledge we have 

 of recent organisms than of the faunas of any, even the most 

 recent, extinct fossil faunas. The evolution kept up its differ- 

 entiation of genera well into the Mesozoic time, when it 

 began to lessen rapidly, and from the Jurassic to the Creta- 

 ceous dropped from 33 to ii in the number of new genera 

 appearing during the periods, and only two new ones ap- 

 peared in the Tertiaiy. 



Comparison of the Rate of Evolution of Generic, Family, and 

 Ordinal Characters. — We may select this division of Brachio- 

 pods for more minute study of the historical laws expressed 

 in the evolution of its successive forms. A study of the 

 curve of results of this series of steps of evolution shows us 

 at a glance that there are, at least, two nodes in the evolu- 

 tion, one culminating in the Silurian and one culminating in 

 the Jurassic. Analysis of the structure of the forms reveals 

 the fact that the evolution has taken place along several sub- 

 ordinate lines, which are expressed in taxonomy by division 

 of the Arthropomata into two primary divisions, called by 

 Beecher orders {Protreniata and Telotrematd), and these 

 again into two groups of families, the Triillacea and Thecacea 

 in the first order, and into the three groups Rostracea, Heli- 

 copegviata and AncylobracJiia of the second order, Telotrcviata. 



Evolution Curves for the Several Families. — Each of these 

 subdivisions was differentiated as early as the Ordovician, or 

 second era, and their climaces are at somewhat different 

 points in the time-scale. 



The first group of families is the Triillacea ; there were 

 no new families of this type initiated after the Ordovician, 

 and no new genera after the Devonian, and the whole group 

 became extinct with the Paleozoic. 



The second group is the Thecacea. Our curve of rate of 



