22 INTRODUCTION. 



4. Circuli. I therefore accept the terms Stomatopora, Prolo- 

 scina, etc., as names for convenient groups, which are not altogether 

 artificial, but which are not genera in the sense in which that 

 term can be used among Echinoidea and Mammals. They could 

 be better described as circuli than as genera. A circulus was one 

 of the small groups of individuals who clustered round speakers 

 in the Roman 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. 



The groups of Cyclostomata appear to me much the same ; in 

 a collection of specimens of Stomatopora, Proboscina, and Berenicea, 

 the vast majority of the specimens can be assigned their position 

 without the slightest hesitation; but occasionally specimens are 

 intermediate between the typical forms, and cannot be so easily 

 placed. Nevertheless, there seems no reason why most of the 

 specimens should not be grouped simply because a few do not 

 exactly fall into line. 



6. SPECIFIC GROUPS AND INDIVIDUAL VARIATION. 



1. The Variation of Zocecia. "When we pass from the genera or 

 large groups of specimens, to the species or smaller groups of 

 specimens, we find that the same variability renders absolute 

 diagnoses again impossible. The specimens may, however, be 

 easily grouped around certain central types. Such groups are 

 based on the characters of the zocecia, viz.: their length and shape, 

 the form of the peristome, and whether the zorecia are crowded 

 or scattered. Variations in the shape of the zoarium are un- 

 important, so long as they do not affect its structure. 



The zooecia, however, themselves vary greatly, and it is there- 

 fore necessary, in order to determine the value of these specific 

 groups, to estimate the range of variation of the zocecia. If this 

 be so great that the same forms are often produced by independent 

 variation from different stocks, at different periods or in distant 

 places, then the time spent in trying to define species or specific 



