﻿1XX1V PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I905, 



from different groups of individuals." To the exposition of this idea, as applied 

 to Bryozoa, eight pages were devoted in the Catalogue. But, in order to justify 

 the adoption of the word eir cuius, the paragraph introducing it laid stress on 

 the subsidiary idea of indefiniteness (p. 22): "A circulus was one of the 

 small groups of individuals who clustered round speakers in the Soman forum. 

 Most of the individuals in the forum were definitely attached to a particular 

 group ; the groups were less crowded around their margins, and between them 

 people were irregularly scattered and crossed from circulus to circulus. They 

 thus prevented any rigid division of the crowd into definite groups." It is clear, 

 however, from Prof. Gregory's own words, that the application of the term 

 circulus to a group of individuals, whether of supposed generic or specific 

 rank, implies not merely that the group has uncertain limits but that it is 

 polyphyletic. 



'A circulus, in short, is an ill-defined assemblage of approximately- 

 isochronous homceomorphs, which it is either inconvenient or impossible to 

 distribute among well-defined homogenetic genera or species.' 



That different organisms are of very different values for purposes 

 of classifying the sediments is well known, and the reasons for the 

 differences of value are in some cases known, in others obscure. 



Those organisms with a long range in time are, of course, mainly 

 valuable for defining considerable thicknesses of strata; while those 

 which had a shorter range may be utilized for separating the smaller 

 divisions. We cannot always infer, however, which organisms 

 will be found to possess a long and which a short range, for in the 

 case of some groups certain genera have a very long range, and 

 others that are closely allied have a brief one : compare, for example, 

 the two brachiopods Distinct and Tr emails . 



Of shallow -water forms, those which belong to the benthos are 

 particularly suitable for determining the facies of the containing 

 deposits. This is especially true of the organisms appertaining to 

 the sessile benthos, and in a less degree to those of the vagrant 

 benthos, for the benthoal organisms existing in tracts where the 

 physical conditions — such as temperature, amount of light, and 

 characters of the sediments of the sea-floor — vary rapidly, are limited 

 as to their horizontal range by the distribution of those conditions 

 which determine their station. As these conditions often change 

 in any area with considerable frequency, the organisms are of course 

 locally useful for chronological division of strata, but, owing to 

 the usually-restricted horizontal range of the forms, they are of 

 little value for correlating strata over wide areas. 1 



1 For information concerning the uses of the planktonic, nektonic, and 

 benthoal organisms of past times to the geologist, see J. Walther, ' Ueber die 

 Lebensweise fossiler Meeresthiere ' Zeitschr. Deutsch. Geol. Gresellsch. vol. xlix 

 (1897) p. 209. 



