FACTS AND OBSERVATIONS. 



xvii 



Like all other Mollusca, these 1658 species did not inhabit the Silurian sea at once, but at 

 distinct times, and separately, in consecutive communities. It is only as genera that this order 

 permeates all stages. 



We shall see, when speaking on this subject, that these populations were renewed at very short 

 intervals, a few survivors being permitted to escape upwards, in ways to be afterwards mentioned. 



In Table I is visible at a glance the opulence and .poverty, more or less, of the different stages. 



We see that the Brachiopoda had a vertical (by strata) maximum and minimum these 

 seemingly capricious fluctuations in quantity being caused by conditions acting upon a tender 

 structure, often incapable of undergoing modification. 



One genus arrives at its maximum in the Primordial, ten in the Lower Stage, none in the Middle 

 Stage, and seventeen in the Upper. The Middle Stage is more abundant in this order perhaps than 

 in any other. 



The seven richest genera (Disclna, Lingula, Orthis, &c.) begin in the Primordial, and push 

 their representatives into the uppermost beds of the period. 



Fourteen genera, all feeble, except Athyris and Spirifer, are first seen in the Lower Stage, only 

 one in the Middle, and seven weak genera in the Upper. 



Three Primordial genera never pass the limits of their native stage. Three other genera belong 

 to the Lower Stage alone. 



The Primordial Stage, the Table informs us, has 126 species of Brachiopoda, in fourteen genera 

 an unexpectedly large supply of food for the carnivorous animals that prevail in this stage. 



The elder Primordial has eleven genera, all, save one (Lingulepis) , crossing the boundary into 

 the stages above. The upper division of this part of the column has six, which do the same. 



The number of Orthides (thirty-three species) in the Primordial stage is remarkably large. 

 That there should be twenty-four species of Lingula we were better prepared for. 



That a Nucleospira (Hall) should be met with in the Primordial is new, but the authority on 

 which the fact rests is too good to allow of its being rejected. 



M. de Koninck, in his Memoir on the genus Chonetes, only enumerates two Silurian species. 

 There are now thirty-one in the ' Thesaurus/ i 



There is some little recurrent movement in the two divisions of this Primordial Stage ; but their 

 organic connexion is slight, being kept up by, at most, two or three species in common. The same 

 species is often found in several beds of the Quebec group ; but few pass the limits of the true 

 Primordial. 



The mixed effects on the number of appearances from horizontal and vertical range are very 

 considerable, but there has not been time to disentangle them. The appearances exceed the species, 

 each genus having its own proportion. In some few there is equality, or an approach to it. 



TABLE I. Proportion of Appearances to Species. 



