GENERAL AND COMPARATIVE STRUCTURE. 33 



but it will give us an obliquely longitudinal section of the coral- 

 lites in the peripheral region of the corallum, where they are 

 bent at greater or less angles away from the vertical. Neither 

 of the above sections, therefore, would give us a cross-section 

 of the tubes in their peripheral region, where they have 

 attained their adult characters, and where it^ is, above all, 

 important to be able to investigate them. In order to obtain 

 what is wanted, it is necessary to make a third section which 

 shall run along a plane parallel with the sin'face, and Just a 

 little belozv it. Such sections we may call tangential, and they 

 are absolutely indispensable to any understanding of the true 

 characters of the great majority of the Monticuliporoids. The 

 precise direction in which it will be necessary to cut any given 

 specimen in order to obtain cross-sections of the tubes in their 

 final and fully developed condition, will necessarily vary with 

 the form of the specimen ; but in all instances the require- 

 ments of the case will be met by sections taken in a direction 

 tangential to the calicular surface, and just below that surface. 

 Moreover, the nearer to the actual surface is the plane of such 

 a section, the greater is the probability of its fully revealing 

 the adult characters of the tubes. 



We may now pass on to consider in greater detail the 

 general and comparative structure of the Monticuliporoids, 

 with special reference to those points which require to be 

 particularly studied in the determination of the different types 

 of this large and variable group. 



I. Forin of the Corallum. — In the first place, as regards the 

 mode of growth of the corallum, and the form ultimately 

 assumed by the colony, we find, as before remarked, great 

 variations, and we are obliged to conclude that the mere ex- 

 ternal shape of the corallum is a character of no classificatory 

 value. It is not that individual species are specially variable 

 in shape, for many types exhibit a tolerably constant form 

 when adult ; but it is the fact that so many structurally diverse 

 species assume the same shape, that robs this feature of any 

 special value that it might otherwise possess. Admitting, 



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