FACTS AND OBSERVATIONS. 

 TABLE C. Progress in British Gasteropoda. 



IX 



We here begin and end with two species, or more truly with four different species ; but the 

 majority of the Gasteropoda are about the middle of the mass of beds, as is usual. The Llandovery 

 beds become rich by borrowing six species from the lower stage. The names of the species 

 referred to in the above Table can be had by reference to Murchison's ' Siluria/ 4th edition, 

 pp. 531 &c. 



In so extraordinarily rich a district as Central Bohemia it is worthy of notice that out of the 

 whole fifty-one genera of this order it has only twenty-three, and these are poor in species. It is, 

 however, the richest area of all, England coming next, with twenty genera ; one only in common, 

 Acroculia. 



The interesting Table D, here subjoined, exhibits the proportions in which the species of the 

 Gasteropoda are found in the four stages of the Silurian epoch, according to our present knowledge. 



TABLE D. 



There are several things worthy of notice in this Time-table of the Gasteropoda. The species 

 augment with the progress of the epoch, those of the upper stage being nearly four times as 

 many as the Primordial. The poverty observed in the Middle Silurian is due to well-known 

 circumstances, and is incident to almost all areas. 



We have in the Primordial stage 121 appearances, which are, except eight, all distinct species. 

 Only one of these species passes into a higher stage, the Pleurotomaria Progne (also of Black-River 

 and Trenton Limestones). 



The genera in the four stages we have adopted are in number 19, 17, 3, and 12 respectively; 

 and the species, taken in like succession, are 113, 329, 85, and 430. 



G 



