GENERAL AND COMPARATIVE STRUCTURE. 31 



obsolete; generally " complete," and approximately horizontal ; 

 sometimes, in a peculiarly modified manner, " incomplete." 

 Corallites usually divisible into two distinct groups, one of 

 large and the other of small tubes, the latter being usually, but 

 not always, more closely tabulate than the large tubes, or show- 

 ing other peculiarities of structure. Surface often, but not 

 always, exhibiting at regular intervals definite areas which are 

 occupied by corallites which are either larger or smaller than 

 the average. These areas may be elevated above the general 

 surface, when they are called " monticules ; " or they may be 

 level with the surface, or slightly depressed below it, when 

 they are called " maculae." 



They^r;;^ of the corallum is extremely variable, though often 

 tolerably, or even altogether, constant for the same species ; 

 the massive, discoid, dendroid, laminar or frondescent, and 

 encrusting types of corallum constituting those most commonly 

 met with. 



The arrangement of the tubes which make up the skeleton 

 of a Montiadipora necessarily varies with the form of the 

 corallum, and the chief variations in this respect will be noticed 

 immediately. There is, however, one character which has such 

 an important practical bearing that it deserves to be noticed 

 in this place. In many Monticuliporoids, namely, though not 

 in all, the tubes which constitute the colony become specially 

 modified in structure as they pass into their final stage of 

 growth, and approach near to the surface. Thus it is very 

 common to find that the corallites in the centre of the corallum 

 are thin-walled, and nearly or quite free from tabulee, whereas 

 in the outer portion of their course, near the surface, their walls 

 become thickened, and tabuloe are abundantly developed. 

 Moreover, it is very usual to find that the colony contains 

 elements, such as small corallites or specially modified tubules, 

 which in general do not extend downwards into the deeper 

 parts of the corallum, but which are only developed in a zone 

 immediately below the actual surface. Hence, in most ordinary 

 types of Monticulipora, such as the great majority of the den- 



