DISTRIBUTION OF TRILOBITBS. 277 



it is a significant fact that in Bohemia, which stands next to Scan- 

 dinavia in respect of number of species, the numerical ratio of 

 Lower Silurian to Upper Silurian forms is as one hundred and 

 eighteen to two hundred and iive, reversing, apparently, the order 

 of development. 



The very limited number of generic forms that pass from one 

 major formation to another is remarkable. Barrande enumerates 

 but seven of the twenty-seven Cambrian genera which pass over 

 into the SiluriaUj and twelve of the fifty-five Silurian genera which 

 reappear in the Devonian. The Carboniferous genera are but three 

 or four in number (Phillipsia, Griflithides, Brachymetopus, Proe- 

 tus).* Of the fifty -five Silurian genera, with three exceptions, all 

 the forms are already represented in the lower division. The 

 number of genera that extend through two or more formations is 

 reduced to two or three (^Phillipsia, Proetus). In their geographi- 

 cal relations it may be said that broad distribution is the rule 

 rather than the exception. Thus, of the forty-two genera of the 

 Bohemian basin thirty are held in common with Sweden, and twenty- 

 four with England. More than one-half (seventeen out of thirty) 

 of the North American genera are also trans-Atlantic forms, and 

 the greater number of these are widely distributed over the Euro- 

 pean continent. It has generally been considered that the most 

 widely distributed genera are those which also have the greatest 

 vertical range; but the exceptions to this supposed rule are so 

 numerous — Paradoxides, Agnostus, Trinucleus, Asaphus — that it 

 may be doubted whether any value is to be attached to it. Nor 

 can it be maintained that in all cases the genera having the longest 

 range are those which have the greatest number of specific repre- 

 sentatives, although this is more often the case than otherwise. 

 Probably the greatest number of species represented in any one 

 genus is exemplified in the case of the genus Dalmanites (Lower 

 Silurian- Upper Devonian), of which there were up to 1871 some 

 one hundred ahd twenty-nine known. The nearest approach to 

 this (some one hundred and fifteen or more) is seen in the genus 

 Asaphus, which is restricted to the Lower Silurian formation. 



The number of species that transgress the boundaries of any 

 major formation is exceedingly limited. Thus, it is very doubtful 



* Professor Claypole has more recently described a species of Dalmanites 

 (Dalmania) from tlie Waverly Group (Sub-Carboniferous) of the United States. 



