FACTS AND OBSEBVATIONS, 



Having deemed the completeness and accuracy of the ' Thesaurus ' itself to be beyond all com- 

 parison my principal duty, and having found this to be a labour, however agreeable, not a little 

 protracted, it has become impossible in the following pages to present more than a simple analysis 

 of a few Silurian orders, and some of the more obvious facts and inferences to be derived from the 

 body of the work. Those, it is trusted, have their interest, and will invite a broader, more leisurely, 

 and more able study than is in my power to bestow. The subjects will fall under the following 

 heads : 



1. The Gasteropoda. 



2. Trilobita. 



3. Echinodermata &c. 



4. The Primordial Stage. 



5. The Bohemian Area. 



6. Universality. 



7. Locality. 



8. First appearance. 



9. Duration or longevity, and extinction. 



10. Migration. 



11. Recurrence. 



12. Divergence. 



Before proceeding to treat on these subjects, it is encouraging to contemplate the progress of 

 Silurian Palaeontology during the last twelve years, as shown in Table A. 



TABLE A. 



This Table is taken from the late Prof. Bromr's well-known and truly admirable Prize Essay of 

 1856, and from the present work. It shows that within the last twelve years the number of known, 

 well-determined species has been more than quadrupled opening to the naturalist nothing less than 

 a new world of life. 



GASTEROPODA. This class of Mollusks consists of 51 genera and 895 species. The geographical 

 summary placed in the body of the Thesaurus contains much information which need not be 

 repeated here. 



Almost all tolerably examined Silurian countries are found possessed of Gasteropoda, but in 

 very different proportions, and of very different kinds. This is in part dependent on the amount of 

 search bestowed. If the results of a resolute search be small, we must, perforce, take for granted 

 that the Gasteropoda are few. Their office is then performed by representatives. 



The following little Table shows in a striking manner the several and extremely various 

 amounts of Gasteropodal species found in some of the great Silurian districts of the earth. 



For further matter see Thesaurus, p. 169. 



Geographical Summary of Species. 



