FACTS AND OBSERVATIONS. 



much in accordance with the size of the country as with the original impress of the Creator and with 

 the nature of the strata. We already have materials from almost all parts of the Silurian scale of 

 rocks to show, with some force (M. Barrande), that life began earlier and more abundantly in the 

 valleys of the St. Lawrence and Mississippi than in Europe. Thus the Primordial Trilobites of 

 these valleys comprehend 208 species, in 40 genera, and very much assembled in two broad quasi- 

 foci, Wisconsin being one, and parts of Lower Canada the other. In Sweden there are in this 

 stage 56 species, in 18 genera ; in Great Britain and Ireland, 33 species, in 11 genera ; in Bohemia, 

 28 species, in 8 genera ; space has little to do here. 



The influence of the Primordial stage on molluscan life, the abundance and typical forms of its 

 species, cannot well be over-estimated and reflected upon. 



The dates of the appearance and disappearance of all the Silurian Trilobites, together with 

 their facies stage by stage, their increment and decrement, should now follow ; but as this is not a 

 monograph, and is intended to be an invitation to labour rather than a completed task, all mention 

 of them will be omitted. For Prague and its environs this has been done admirably. 



The number of species now known and registered is 1677, and their appearances in the several 

 regions are 2169, the former being rather less than four-fifths of the latter. The difference (492 

 appearances) becomes the measure of their dispersion; and it is less than that of most of the 

 molluscan orders of this period. 



Very unexpectedly the species of this order in North America are much fewer than in Europe, 

 being only about one-third of the whole. This may, by possibility, arise from the prevalence of 

 sandstone over limestone in North America. In New York State the limestones are only 600 feet 

 thick, but the sandstones are 3000 feet thick ; and there is little reason to think things are different 

 in other parts of that continent. In both cases neither the superficial areas nor the research 

 bestowed upon them differ much. 



The geographical summary here subjoined gives the number of species of Trilobites found in 

 each of the forty-eight districts, as far as is known. To most of the areas interesting details are 

 attached, which are to be found in the ' Thesaurus ' and elsewhere. 



The totals of species and appearances in America and Europe are inserted separately. 



TABLE F. TRILOBITA (Geographical Summary). 



The great total for both hemispheres is 1677 species. 



