chief Oeneric Tyjpes of Paloiozoic Carols. 457 



ever, we do not necessarily contend that these groups attain to 

 the conventional rank of genera. Believing that the terms 

 " genera " and " subgenera," as applied to inosculating groups 

 like the one we are now considering, possess a purely artificial 

 value, we are not concerned to contend for the employment of 

 the one term rather than the other. We are, however, of 

 opinion that the use of subgenera in pal^eontological inves- 

 tigations has proved itself by experience rather a hindrance 

 than a help to progress ; and we have therefore preferred to 

 leave these groups in the position of " generic types " until 

 some strict and explicit definition of the terms " genus " and 

 " subgenus " may fortunately be agreed upon. 



The genus Clisiophyllum is stated to range from the Upper 

 Silurian to the Lower Carboniferous. The true afiinities of 

 the Silurian forms, however, cannot be regarded as altogether 

 free from doubt ; and it is singular^ if they have been rightly 

 determined, that no representatives of the genus (as here de- 

 fined) have hitherto been detected in the richly fossiliferous 

 deposits of the Devonian period. 



Genus Dibunophyllum, Thomson and Nicholson. 



(Sir, twice ; /Sowvoy, hill ; (^vXXoj/, leaf.) 



Oen. char. Corallum simple, turbinate, or cono-cylindrical. 

 Epitheca complete, thin, with numerous encircling striaj and 

 annulations of growth. Calice of variable depth, usually 

 shallow, exhibiting centrally at the bottom a rounded emi- 

 nence, which is slightly raised above the inner margins of the 

 primary septa, and is always divided by a distinct longitu- 

 dinal mesial line separating the eminence into two equal 

 halves. The calicine eminence is formed partly by a median 

 elevation of the tabula, and partly by a series of somewhat 

 irregular vertical lamellse, which are united with the inner 

 edges of the primary septa by subconvolute plates, but are 

 altogether independent of these structures, and appear on the 

 surface of the boss as so many ridges radiating from the mar- 

 gins of the central area to the sides of the mesial crest. Septa 

 well developed, generally of two orders ; the secondary septa 

 short or wanting, and the primaiy septa invariably falling short 

 of the central area. Interseptal dissepiments scanty and 

 remote in the intermediate area between the inner ends of the 

 primary septa, but very abundant in the external area, where 

 they form a dense vesicular tissue, appearing in longitudinal 

 sections as minute lenticular cells arranged in oblique rows 

 directed upwards and outwards. Longitudinal sections also 



