Vol. 69.] THE GENUS AULOPHYLLUM. 69 



All things considered, the intensity of the septal lamellae affords 

 the most satisfactory plan of identification. It is, however, often 

 difficult to compare individuals of different types ; and, moreover, 

 the number of lamella? (with which intensity is really synonymous) 

 that would appear loose in a large central column would appear 

 denser in a smaller one. 



VII. Classification and Consideration oe Time-VapvIants. 



(1) Recognition of Single Species with several Mutations. 



I propose to assign all the ' species ' described by Thomson and 

 -others to the one species, Aulophyllum fungites, but recognize 

 several mutations according to the stage of development reached. 



(2) Mutations. 



The mutations of the genus and the relations existing between 

 them are arranged in the following table : — 



Upper and 



Middle 



Dibunophyllum 



Zones. 



(d;-d 3 -) 



Aulophyllum fungites 

 mut. pachyendothecum. 



Aulophyllum fungites 

 muts. towards pachyendo- 

 thecum. 



Aulophyllum fungitc 

 mut. cumbriense. 



Lower 



Dibunophyllum 



Zone. (D r ) 



A u lopliyllum jungiies 

 mut. redesdalense. 



Aulophyllum fungites 

 mut. tenbiense. 



(3) Forms from the Lower Dibunophyllum Zone (Dx). 



Aulophyllum fungites mut. redesdalense. 



The distinguishing characters of A. fungites mut. redesdalense 

 are the paucity and irregularity of the principal lamellae and the 

 general looseness of the tissue, resulting in a loose design, of 

 which the ' circular cells ' are the most conspicuous feature (PI. V, 

 fig. 6). In short, the adult character of the central column is that 

 of Stage D of A. fungites mut. pachyendothecum. 



A. fungites mut. tenbiense. 



This mutation is also distinguished by the paucity of the prin- 

 cipal lamella? ; but these ramify over the interior of the central 



