290 



GEOLOGICAL BIOLOGY. 



that the Helicopegmata lived on to the middle of the Mesozoic, 

 and, third, that most species have a life-period of a third or 

 half of the duration of the whole Silurian time, it is no exag- 

 geration to say that the evolution of these modifications of 

 the brachidium was, relatively to all laws of organic change in 

 geology, extremely rapid. 



Rate of Initiation of the Genera of Helicopegmata. — If now we 

 reduce the facts of generic differentiation to graphic form, we 

 find that the sudden or rapid differentiation is a fact, and is 

 not due to imperfect evidence. Considering, as in previous 

 cases, classification to be a mode of expressing degrees of dif- 

 ference, we may rely upon the mathematical relations of initi- 

 ation of the groups of equal rank as an expression of the rate 

 of initiation of new characters in general, or an approximate 

 measure of the rate of geological evolution. 



TABLE EXPRESSING THE RATE OF EXPANSION OF THE FAMILY, SUBFAM- 

 ILY, AND GENERIC CHARACTERS OF THE HELICOPEGMATA. 



Hblicopkgmata. 



Families . 



Subfamilies. 



Genera 



The Helicopegmata as a suborder is found to be repre- 

 sented in three family types of structure: one of these ap- 

 peared first in the Ordovician, in a single subfamily, a single 

 or possibly two genera, and but few species. At the opening 

 of the next era, the Upper Silurian, the other two families 

 appear, and seven out of the known twelve subfamilies were 

 .initiated. 



