276 GEOGEAPHICAL AXD GEOLOGICAL DISTEIBUTION. 



species of trilobites found in tlie inferior division of tlie Lower 

 Cambrian of Britain (the Longmynd and Harlech groups) is about 

 ten, representing six genera — Agnostus, Conocoryphe, Paradox- 

 ides, Microdiscus, Pahieopyge, Plutonia — and of the entire Cam- 

 brian series one hundred, distributed as follows : 



Upper Tremadoc 13 (including 3 from the Upper Lingula). 



Lower Tremadoc 19. 



Upper Lingula 30 ( " 5 " Lower Lingula). 



Lower Lingula 22 ( " 14 " Menevian). 



Menevian 33 ( " 4 " Longmynd). 



Longmynd and Harlech 10. 



Prom the Primordial zone of Bohemia (Cambrian) Barrande 

 recognised in 1871 only twenty-seven species, or but little more 

 than one-tenth the number (252) which he claimed for the Cam- 

 brian deposits of the world generally. This paucity, as compared 

 with the richness of the British fauna, is the more surprising when 

 we consider how largely in excess of the insular forms are the Silu- 

 rian species from the same region. Tiie Bohemian basin contained 

 at the period stated about three hundred and twenty species, 

 whereas the total number of species recognised at about the same 

 time from the entire British Paleozoic series of deposits — i. e., from 

 the Cambrian to Carboniferous inclusive — only slightly exceeded 

 two hundred and twenty. 



Of the seventeen hundred species, representing seventy-five 

 genera, tabulated by BaiTande for the world at large we find 

 (using his data) two hundred and fifty-two relegated to the Cam- 

 brian formations, eight hundred and sixty-six to the Lower Si- 

 lurian, four hundred and eighty-tv/o to the Upper Silurian, one 

 hundred and five to the Devonian, fifteen to the Carboniferous, 

 and one to the Permian.* While these figures would indicate a 

 pronounced culmination of the group in the Lower Silurian period, 



* The number of species lias been very materially increased since the 

 publication of Barrande's paper, but his e.-itimates will still serve as a proper 

 ba,-is for the computation of ratios. The American Carboniferous species 

 seem to fall not far short of a dozen by themselves. Considerable uncer- 

 tainty still attaches to the stratigraphy of the forms that have been referred 

 to the Permian of Germany and the United States (Guadalupe Mountains, in 

 Texas and New Mexico), and it is generally assumed that there are no Per- 

 mian species at all. 



