CXXll INTRODUCTION. 



and clearly discrimiuating, the more constant and perma- 

 nent forms, with due attention to their evolutional his- 

 tory and varietal tendencies, than by merging them in 

 genealogical groups the composition of which must, in 

 many cases, be largely speculative. 



One of the most important features of Prof. S mitt's 

 system is the place which he assigns to the zooecium in 

 the construction of families and genera. The mere mode 

 of growth he treats as a perfectly subordinate character, 

 and bases his divisions chiefly on the essential element 

 of the structure, the cell *. This revolutionary step in- 

 volves the breaking-up of a large proportion of the older 

 genera and the wide dispersion of forms hitherto most 

 closely associated ; but there can be no doubt that it is a 

 necessary preliminary to the introduction of a natural 

 system. The varieties of habit which have been made 

 the criteria of genera, may occur within the limits of a 

 species. It is not the mode in which the cells combine, 

 but the cell itself, that is the true test of relationship 

 and the essential basis of a natural group. Prof. Smitt 

 has remarked, "Sempet^ videbimus, zibi forma zocecii eadem 

 sit, ceteras dissimilitudines altero vel altera modo evane- 

 scere." 



Retaining Mr. Busk's three principal divisions of the 

 Gymnolcemata, he arranges the Cheilostomata under four 

 groups : — the Flustrina, with quadrate cells, the front sur- 

 face of which is flat and equals the area of the primitive 

 aperture ; the Cellalarina, with corneous or corneo-calca- 

 reous infundibuliform cells, the inferior portion of which 



* 01' eoui-Bo, ill practice, this principle applies chiefly to the Cheilosto- 

 mata. Amongst the Cyclostomafa there is great nnifonuity of strnctnre 

 in the zooecinm ; and in the case of the Cfcnot<to)i/ata other important cha- 

 racters are available. 



