184 Barrande— Origin of Paleozoic Species. 
by its completely contracted aperture, the more ancient form 
corresponding wi e ill, we observe one 
important deficiency, namely, the total absence of the more 
simple forms of the order, 7. e, the Ascoceratidw. The number 
of species derived from these 12 primitive types is about 166. 
* * This number represents about one-third of the 478 forms 
which characterize the second fauna in all Silurian coun- 
tries. Thus, the order shows itself already largely developed, 
in generic types and specific forms, upon the horizons where 
we observe the most ancient traces of its existence. We ought 
also to note the important fact that, during the first epoch, the 
number of migrating species, or those common to many coun- 
tries upon the grand northern zone of Europe and America, 
does not constitute one-fourth of the sum total of existing 
forms. There is, moreover, no form common to these northern 
especially each of the great 
Zones, possesses many contemporaneous t which exclusively 
belong to it. But itis écially- the listribution of specific forms 
which offers us one of the most remarkable examples of ocali- 
zation. In fact, among more than 240 species already know? 
in the whole of the Primordial fauna properly so called, the 
number of those which are common to two countries geog raphi- 
cally separated is very . Thus the circumstances which 
seemed the most inexplicable, in the first appearance of the 
