I \ 1 RODUCTION. CXX1X 



fronds. Many cases of the same kind occur. Smittia 

 Lundsborovii&rid. Lepralia foliacea exhibit the same diver- 

 sities of habit, the crustaceous predominating in the 

 former and the foliaceous in the latter. A considerable 

 number of species are now known in which the same cell 

 is associated with very various modes of growth. The 

 genus Porella of the present work embraces species which 

 exhibit the most striking differences of habit ; but they 

 all possess zooecia which are both essentially identical in 

 the adult state and pass through the very same course of 

 development. So far as the cell is concerned, there are 

 none but specific distinctions between them ; and we cannot 

 doubt that it is more philosophical to unite them in one 

 genus on the strength of their structural similarity, than 

 to separate them for their diverse habit. 



I should lay it down, then, that in the formation of 

 families and genera in this division, and especially in the 

 Escharine group, the characters of the cell must be taken 

 as the basis, and a secondary place be assigned to mere 

 zoarial habit. 



What, then, are the most significant features of the 

 zooecium for classificatory purposes? Form, superficial 

 sculpture, the presence or absence of spines or other ap- 

 pendages, these are generally too variable and inconstant 

 to yield any sure criteria. But we may find such in the 

 structural peculiarities of the cell as, for instance, the 

 modifications of the aperture, the degree in which the 

 primitive opening is preserved or obliterated, the ribbed 

 condition of the front wall (as in Membramporella and 

 Cribrilina), the chambered condition of the cavity (as in 

 Steganoporella) , &c. One of the most constant features 

 of the zooeciura, too, is to be found (as noticed long agq 



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