FACTS AND OBSERVATIONS. 



That the very earliest phase of this epoch should contain so large a body of Gasteropoda 

 implies a large amount of maturity in the conditions of the time, as well as of adaptation of organic 

 structure. We also find them even then associated with Cephalopoda of high rank, such as Clymenia 

 and Orthoceratites ; while the Gasteropoda maintain themselves vigorously, with organs of great 

 power. We must, perforce, be greatly struck by this outburst of new and complex forms of life 

 after an apparently long arrest of creative energy. (See p. xx, on the Primordial Stage.) 



Nearly all of these many Primordial Gasteropoda are natives of North America, the exceptions 

 being only five, i. e. Ophileta compacta and Euomphalus matutinus in N.W. Scotland, Acroculia 

 Cantabrica, A. sp. ind. of Spain, and Helicotoma Anglica from South Wales. Bohemia has not 

 a single Primordial Gasteropod, remarkably numerous though they be higher up. Only two small 

 genera stop within the Primordial beds ; most of the others pass, in the form of new species, freely 

 up to the summit of the epoch. 



The Table E, subjoined, shows the comparative numbers with which the species of this class 

 people the sediments of the Silurian epoch. 



TABLE E. 



The appearance of the same species in more than one sediment (which has been called 

 " divergence") is frequent in Wales; for eighty-four species make 158 appearances in more or fewer 

 of the ten kinds of sediments of the Table; many frequent more beds than two. There are 115 

 appearances in calcareous beds, but only 43 in non- calcareous beds. We see very few Gasteropoda 

 in limestone which is pure, or nearly so ; and this holds good with most other Silurian Mollusca. 



In the State of New York the preference of Gasteropoda for sediments containing some lime 

 is eight times as strong as for non-calcareous beds t ; that is, out of ninety-three Gasteropoda, 

 eighty-three are in calcareous beds. 



Of the ninety-seven Silurian Gasteropoda known in Britain and Ireland in 1868, twenty species 

 are recurrent ; but six of these are only transferred from the Llandovery stage to the Wenlock. 

 Euomphalus sculptus, however, and Platychisma simulans are in four stages (Caradoc, Llandovery, 

 Wenlock, Ludlow) ; Euomphalus funatus and Holopella cancellata are in three, the rest being only 

 in two stages and thus indicating a weak power of recurrency. 



TRILOBITES. Of the 119 genera which form this order, 74 make their first appearance in 

 the Primordial stage of the epoch, and they present at one period or other of their existence 998 

 species, as we are taught by the ' Thesaurus/ 



These Primordial genera are nearly two-thirds of all those contained in Silurian beds. Forty- 

 six of these genera never leave this the earliest of the four stages. Not one of their 235 species is seen 

 in Caradoc, Pleta, Trenton, or any other part of the epoch ; but twenty-seven genera push upwards, 

 arid always by new species, leaving the old ones behind. These genera in their transit through the 

 successive stages become exceedingly rich in species, and several are rich within the Primordial 

 limits (Bathyurus, Agnostus, Olenus, Dikelocephalus , &c.). 



A Primordial species usually has a considerable vertical range within its native stage, and 

 there only. 



The Primordial Trilobites vary in amount and kind with the country examined, and not so 

 * See Hutchison's ' Siluria,' 4th edition. t Journ. Geol. Soc. Lend. vol. xv. p. 308. 



