70 Mr. J. Thomson and Dr. H. A. Nicholson on the 



nor does it show a regular series of lines representing the cut 

 edges of the vertical lamellse, and radiating from the circum- 

 ference to the centre, such as is present in both of the above 

 genera and also in Clisiophyllum. On the contrary, we find 

 in Rhodophyllum (PI. I. figs. 1-3) a series of lines, of a wavy 

 and irregular character, but upon the whole twisted in a con- 

 volute or contortive manner round an imaginary centre. These 

 spirally twisted lines are formed partly by the cut edges of 

 the vertical lamellse of the central area, and partly by the cut 

 edges of the tabula of the same region. In fact, the vertical 

 lamellae of the central area may be regarded as being com- 

 paratively rudimentary, as compared with their development 

 in Clisiophyllum, Dibunophyllum, and Aspidophyllum. 



The four groups which we have now considered under the 

 names Clisiophyllum, Dibunophyllum, Aspidophyllum, and 

 Rhodophyllum form four natural assemblages which are most 

 intimately related in the fundamental plan of their organization. 

 Though their morphological type is substantially the same, 

 and though transitional and intermediate forms are not wanting, 

 each of these groups is nevertheless distinguished from the 

 others by special structural peculiarities, of such constancy 

 and of such importance that they may be best indicated by 

 special titles. To this conclusion we have been led after a 

 careful examination and comparison of several hundreds of 

 cut and polished specimens, and of thin sections prepared for 

 the microscope. That the course which we have been led to 

 adopt may not recommend itself to others is quite possible. 

 We are most willing to admit that it must remain in tlie 

 meanwhile a matter of opinion whether such diflferences as 

 we have endeavoured to indicate as existing amongst the 

 Clisiophylloid corals are of generic or subgeneric value, or 

 even whether each type might not rather be considered as 

 constituting a single, extremely variable species. This point 

 can only be decided by the opinion of each individual observer ; 

 and its decision will inevitably depend more or less upon the 

 views which each individual may hold as to what ought to be 

 understood by the terms "genera," " subgenera," and "species." 

 With regard to the actual existence of the different groups which 

 we have described, we can confidently appeal to the facts of 

 nature : whether our interpretation of these facts be correct or 

 not is a matter of comparatively small moment. 



As the series of related and inosculating groups, of which 

 Clisiojjhyllum is the central form, is one of great complexity, 

 and presenting unusual difficulties in its study, we introduce 

 here diagrams by which the leading differences of these groups 

 maybe readily apprehended (see p. 71). In all these forms (save 

 ^ the aberrant genus Carcinophy Hum) the structure of the external 



